We have upgraded several V2110s and C4110s (more than 10 pair total) and the install has gone smoothly most of the time. The only "issue" we've seen is for 2 different customers (Each had 2 V2110s in Availability), after the first controller is upgraded, the backup tunnel went down to the foreign APs.
The foreign APs showed as up after about 10 minutes. When updating the other controller in the pair, the same thing happened - the APs foreign to that controller (homed to the 1st controller) showed as down for about 10 minutes. The APs are still functional, and likely it is just some sort of glitch, but it was something that stressed us out for a bit when it first happened.
Since everything comes up, and it only seemed to be these 2 pair of controllers, we figured it was mostly an isolated issue.