Upgrade: Juno -> Pike (or maybe later)

Hey -- I've run a number of OpenStack clouds from multiple vendors for several years, now. But I've *never* done an upgrade. I've read some of the docs, but they don't really make it clear (at least, the ones I've thus-far found): * If going from Juno to something pretty recent is even feasible * If I have to do intermediate steps along the way * How likely it is to "just work:" complexity, common issues, etc. The fact that there's a clearly codified fallback procedure, while an inherently good thing to have, does concern me some. Any insights would be much appreciated. Thanks! -Ken