We saw it with TBC, we saw it with WotLK, and we'll assuredly see it again with Cataclysm. Players opt to try out the newly released race/class, and some simply just want a change. While there will be the eventual opportunity for players to race-change their mains to one of the new races (goblin/worgen), some will simply wish to reroll and level afresh, be it for story, role play, or just "clearing the slate."
Guilds need to be looking ahead to this, because it means a changeup of their rosters. An unprepared guild could find itself flailing for raiders to fit specific roles, and could easily disband over time as the players seek out new, stabler homes. Is your guild ready for this?
- Start up a discussion. Even if you're just opening the door for some "alt nights" to play out the new races, you need to start somewhere. Many raiders may not say anything about their plans unless asked.
- Talk out roles. Is your tank suddenly becoming a healer? Your healer going dps, etc? What will that do to your roster? Is there someone else interested in switching to that empty slot, or do you need to keep an eye out for recruits? Is the new role one that they have played before and have some experience with, or is it just a wild guess that they want to try out and may not fit with them, leading you to a longer wait at 85 as they figure out what they want to do?
- Talk out races. Is the player that wants to switch to the new race able to wait and perform a race-change on their main, or will they need time to re-level and, possibly, learn a new class or spec?
- Talk out classes. Is someone disillusioned with the class that they play? Do they have an alt that they enjoy more? This is a chance for them to change it up and get out of the rut with their current class, which will in turn keep them interested and enjoying the game.
- Talk out mechanics. Not everyone follows blogs or news sites, and the surprise of the changes may be waiting for them as late as them logging into the live update: surprises like that, particularly if they're rolled out before the expansion is released, can put a dent in your ICC raid nights while the uninformed player tries to quickly make heads and tails of it all.
- Talk out professions. Notably for Cataclysm, every player should be made aware of reforging (altering gear stats slightly with tailor/leather/blacksmithing) and the Archeology secondary profession. This can also include individuals' profession changes, such as the slew of jewel-crafters that emerged in TBC, and inscriptionists that came out with WotLK; if your favorite enchanter is planning to level a new profession, you may have a problem. Motivating the guild to stock up ahead of time on materials for leveling professions into the expansion is also a very good idea.
- Talk out schedules. I'm talking long-term vision here: is someone starting up college? Graduating college and starting a new job? Picking up a summer job, or taking evening classes? Not allowed to raid during the fall? Picking up a sport or martial arts? Having a baby? Moving to Antarctica (or just a couple timezones)? What do those players wish to do about raiding, then?
For a non-druid example, Mutilate rogues became viable raiding specs from the laughable fail-specs that they were, and for a time they enjoyed a WotLK single-target dps spotlight of overpoweredness. What TBC rogue would've expected that?
Be ready for class changes, and be ready for your guild to change things up on you. Don't let your guild get blind-sided with Cataclysm :)