It's officially the end of the expansion, because I'm tanking again. Every expansion, this happens. I start off tanking, something happens and I switch to DPS, and then by the end of the expansion I'm tanking again. The best part is that it usually happens in the middle of working on heroic progression, meaning that I'm suddenly tanking the hardest fights in the game. Usually without having ever tanked them before, in fact. Last night for instance I found out that I am much better at doing Heroic Norushen if I go down to kill the big add with some Vengeance built up, for how much it helps with Shield Barrier if I mis-time and get hit by the big attack.
My gear is adequate - about ilevel 566, with quite a few heroic pieces - but it's still a learning curve and one that's sometimes fairly hard to adjust to, and not just for me. This is a raid we've been clearing weekly, and suddenly here's a new tank who doesn't know what's going on as well as the previous tanks did. Combine that with my general sense of perfectionism (I do not like making even the most understandable mistake) and it can be pretty stressful.
But sometimes it's necessary -- your group has all the healers it needs, and you volunteer to DPS. You've been playing a hunter, now you're on your priest healing instead. (Hi, Final.) You're a tank but you're burned out and you need a change of pace. So, how can you deal with this, both as a player and as a group with a player switch? While I'm not pretending to having any sort of universal answer, here's a few things I've noticed.
First off, there's a certain amount of slack you're going to just have to extend. If you're the player switching, you have to extend it to yourself -- you're going to make mistakes playing an unfamiliar role, doing things on fights you never had to do before. These mistakes aren't bad - they're how you learn what you're doing. You're in the unenviable position of not just learning the role you'll be playing, but to some degree unlearning the role you were playing - depending on the fight it can end up being very different indeed. As for the group, you'll need to extend your patience. Yes, this is a fight you've already seen and done over and over again and now suddenly you're wiping because someone else didn't know how to do something -- it's annoying. Often, you just want to get through it and on to the next boss. But it's simply the best way for people to learn what they're doing - no amount of prep time can replace seeing the encounter and realizing its nuts and bolts.
That being said, you're not excused from doing that prep. Make sure the gear you'll need to perform the role is up to date. Make sure to find out how the fight differs for your new role - don't just assume you've got it all down because you've been there before. Be cognizant of how different classes perform in similar roles -- does your class have any specific tricks (like Spell Reflect on H. Norushen) to make the fight easier? Make sure you have the right consumables - don't just show up with all the stuff you farmed for DPS if it's not the right stuff for healing/tanking (and vice versa, of course).
Also, if there's anyone else in the raid who can actually advise you on what you're going to do - an experienced tank, for an example, who's done more of the fights than you have or a DPS who can walk you through where to stand - then take advantage of them. And for a group, if you are that person, it's useful to offer that advice. It benefits you as much as it does the person getting the advice, because if they learn what they're doing faster, everything goes smoother, stuff dies and the group wins. Don't go crazy on the advice offering, though - too much information can swamp a person and leave them feeling confused.
Ultimately, role changes are a combination of stressful and invigorating - in a real way, they make a raid an entirely new experience. I can't say I recommend it -- I probably wouldn't be doing it myself if it wasn't necessary -- but it's definitely made raiding more engaging at the moment. It can, and should, be approached from a position of accepting what you know and learning how to get better.