To be completely honest I think a good amount of raiding guilds are losing members come Wrath. The expansion is a chance to wipe your slate clean and start new. Most of the top level guilds are even disbanding in the days leading to wrath.
Losing members comes with the job. If you aren't prepared to lose/gain good players, GM is not the job for you. Just wait it out until Wrath, there will be people looking to raid
