Since flex/normal guilds won't need to make any changes (in fact the changes for normal will only be good as you won't need to worry about bench players) I'm excluding them. But for heroic guilds transitioning to mythic 20m I have this question:
What is your guild doing now to prepare for it when it comes?
For 10m guilds are you biting the bullet and going 25 now? Or are you going full force and going to kill heroic Garrosh (or as far as you can get) before change? For mid tier heroic guilds, do you plan on gradually going to 20m during WoD and essentially "10m" heroic in WoD and get to 20 before mythic?
For 25m do you plan on cutting people or just going to have a large bench and see who is left after new xpac and aren't going to do anything different?
My 25m guild probably will just have a large bench and assume some will leave when new xpac comes.