I got a 2015 15" MBP running Sierra - no complaints really - it's a solid OS and does everything I want. I'm just very intrigued by the new features in Mojave, mostly Dark Mode. My question is - if I do a time machine backup today (on Sierra), and then upgrade to Mojave - would it be easy to revert back to Sierra if I dislike Mojave? Is the downgrade a simple process, or does it include a lot of grunt work?