Moving from ISO 9001:2008 to 9001:2015
Naturally, the ISO has information devoted to this change and a good staring point is their landing page on this topic [here]. On that page, they share a YouTube video from a Google Hangouts session on this topic. That video is about 33 minutes long, worth the view in my opinion, and available [here]. This ISO landing page has a nice chart with the evolution of this standard as well as some information about the standards development process - if you are interested in that kind of thing.
To read specifically about the move from 2008 to 2015, I suggest the ISO published brochure on this topic. See that [here]. It is short (only 8 pages) but provides some good high-level info on this topic.
ISO 9001 is the responsibility of ISO Technical Committee No. 176, sub-committee 2 (a.k.a. "ISO/TC 176/SC 2"). To get a detailed information on the change and committee actions relative to this change, see their landing page [here].
The Technical Committee information may be a bit overly technical, even arcane, for those looking to implement ISO 9001:2015 or manage an "upgrade" from 2008 to 2015. For that purpose, I suggest the American Society of Quality (ASQ) page devoted to this topic at [here]. In addition to useful reference material, this page has links to more "how to" information about ISO 9001:2015.
Hope that helps others. In return, would be eager to hear of resources professionals working in this area have to suggest. Please comment on this post or contact me using the information [here].