The  two standards cover similar topics, but  there are some important differences.  Also, here are some audit necessities.

Structure of the standard -  ISO 9001 2008 had five main sections (4 to 8) and ISO 9001 2015 now has seven main sections  (4 to 10).

Context of the organization - The new standard expects you to understand your organization's context before you establish its QMS.    When ISO 9001 2015 asks you to understand your organization's context, it wants you to consider the external and internal issues that are relevant to its purpose and strategic direction and to think about the influence these issues could have on its QMS and the results it intends to achieve.  ISO 9001 2015 asks you to understand your organization's  external environment, its culture, its values, its performance, and its interested parties before you develop its QMS. Why?  Because your QMS will need to be able to manage all of these influences.  And once you understand all of this, you're expected to use this special insight to help you define the scope of your QMS and the challenges it must deal with. While this will certainly help ensure that organizations develop unique quality management systems that address their own needs and requirements, doing all of this could be quite a challenge for some organizations.

External and internal issues - how can these effect your organization and who/what are they? You must identify them. How did you come to choose these as external and internal issues?

Documented information -The new ISO 9001 2015 standard has also eliminated the long standing distinction between documents and records. Now they are both referred to as “documented information."  According to ISO's definition, the term documented information refers to information that must be controlled and maintained. So, whenever ISO 9001 2015 uses the term "documented information" it implicitly expects you to control and maintain that information and its supporting medium. So, whenever the new standard refers to documented information and it asks you to maintain this information, it's talking about what used to be referred to as procedures, and whenever it asks you to retain this information, it's talking about what used to be called records. So sometimes it must be maintained and sometimes it must be retained.

So, while the definition of the term "documented information" abandons the distinction between documents (or documented procedures) and records, through the use of the words "maintain"and "retain" and because of what this means (according to Annex A) the main body of the standard actually restores this distinction.

Risk-based thinking According to the new standard, “risk-based thinking has always been implicit in ISO 9001”.  According to this perspective, ISO 9001 has always been about anticipating and preventing mistakes, which is what risk-based thinking is all about. That's why we train people, why we plan our work, why we assign roles and responsibilities, why we validate and verify results, why we audit and review activities, and why we monitor, measure, and control processes. We do these things  because we want to prevent mistakes. We do them because we're trying to manage risk. So, if we think of risk-based thinking in this way, it's always been an inherent part of ISO 9001. Before it was implicit; now it's explicit.

So what kind of thinking is risk-based thinking and how is it applied? What does the new standard expect organizations to do?

The new standard expects organizations to identify and address the risks that could influence their ability to provide compliant products and services and to satisfy customers. It also expects them to identify and address the opportunities that could enhance their ability to provide compliant products and services and to satisfy customers.

The new ISO standard also expects organizations to identify the risks and opportunities that could influence the performance of their quality management systems or disrupt their operation  and then it expects them to define actions to address these risksand opportunities. It then further expects them to figure out how they're going to make these actions part of their QMS processes and how they're going to implement, control, evaluate, and review the effectiveness of these actions and these processes.

While risk-based thinking is now an essential part of the new standard, it does not actually expect you to implement a formal
risk management process nor does it expect you to document your risk-based approach.

Requirements and exclusions  Section 1.2 of ISO 9001 2008 says that organizations may exclude or ignore product realization requirements (section 7) if they cannot be applied and if doing so doesn't interfere with its ability or responsibility to meet customer and legal requirements. The new standard takes a similar approach but, instead, seems  to apply this thinking to all requirements.

Section 4.3 of ISO 9001 2015 says “The organization shall apply all the requirements of this International Standard if they are applicable within the determined scope of its quality management system”.  So once you’ve determined the scope of your QMS, ISO 9001 2015 says that every requirement must be applied within the boundaries defined by your statement of scope if it applies in your case.

However, while the new ISO 9001 2015 standard says that every requirement must be applied, section 4.3 and Annex A5 also says that any requirement may be excluded if it cannot be applied, if you can justify and explain why it can’t be applied, and if excluding it does not undermine your ability or responsibility to ensure that products and services are in compliance.

So, the message is clear:  if a requirement can be applied you can't just ignore it. You must apply it. And if you really can’t apply it, you better be able to explain why not.

Objects, outputs, products, and services The definition of the term “object” is new. The introduction of the term “object” to mean anything conceivable or perceivable and its use in various definitions (quality, design and development,innovation, review, traceability) seems to suggest that the new ISO 9001 standard can be applied to any object whatsoever. In theory at least, this greatly expands its scope.

What ISO 9000 2008 used to call a “product” the new standard now calls an “output”. The two definitions are the same. Since the term “output” was not defined in 2008, this shift in terminology suggests that the process approach is now even more central to the new standard. And to further complicate things, the old definition of “product” has now been split into three separate definitions for the terms output, product, and service. “Output” is the general concept since both“products”  and “services” are now thought of as “outputs”.

Other clarifications and modifications While the previous changes could be the most important ones, the new standard has also clarified some concepts and modified others.  Some of these changes are listed below.

The old standard said that a “service” was a type of “product”.  Now, the phrase "products and services" is used throughout the new standard and the term "service" has received its own definition. This should help make it clear that ISO 9001 2015 applies not only to manufacturers but also to all types of service providers.

What used to be called “customer property” has been modified and greatly expanded to include products, services, and processes belonging to all types of external providers (including customers). The new standard now expects you to control externally provided products and services if they are included in your products or services or if they are provided directly to customers.

The old definition of “continual improvement” has changed. When ISO 9001 2008 asked you to make continual improvements
it was asking you to improve your ability to fulfill requirements. Now, ISO 9001 2015 says it means enhancing performance
(getting better results).  This is an important shift.

According to the new standard, organizations must now identify ,acquire, and share the “knowledge” that personnel need in order to support process operations and achieve conformity of products and services.

The old concept of “product realization” is gone. Most of the material in the old product realization section has been modified
and moved to the new ISO 9001 2015 section on Operations.

The term “management representative” has been dropped. The management duties and responsibilities that were previously
assigned to someone called a “management representative” may now be assigned either to one person or to many people.

"Preventive action" has also disappeared. It’s been replaced by "risk-based-thinking", evidently because both approaches try
to achieve the same thing. Both try to prevent future problems. Once you introduce risk-based thinking, you no longer need a
separate clause on preventive action. It's redundant.

While the old standard asked you to use monitoring and measuring “equipment”, the new standard refers to monitoring and measuring “resources”. This is a more flexible approach to monitoring and measuring because it recognizes the fact that these activities can often be carried out without the use of equipment.

Differences Between ISO 9001 2008 vs ISO 9001 2015