If you already have a quality management system in place such like ISO9001:2008 then you have a lot of the foundations in place and will go through what is termed as a transition to ISO9001:2015. This will generally happen after you have gone through what BSI term a readiness review. This may be termed a something else depending on what body you use, my experience is with working with BSI.
I will go through in more detail my experience of a readiness review in another post. For now to aid in the transition BSI have created the following document that explains transitioning that may be of use.