In my 30+ years of experience in technology, I have migrated Windows 95 to XP, Windows NT to Windows 7, and now Windows 7 to Windows 10. And they all had one thing in common: I had to come in and rescue the migration projects many times because, initially, in peoples’ heads — even if they are technologists — OS migrations are simply thought of as clicking "Next" a few times.
However, the opposite is true. In my opinion, upgrading to a new operating system in a large organization is one of the most difficult projects you can undertake. It touches every single person, application, and technology in the organization. You have to be extremely methodical and prepared in everything you do to finish the project successfully. Otherwise, your timeline, budget, and everything very quickly get away from you.