In my opinion I dont think it will change a lot. Wait about 12 months from WotLK release, then check your realm progression. I'll bet you a dollar that players currently in T5/T6 will be the ones raiding in WotLK. Guilds I think will be either 10 man or 25 man, not both. If a 25 man raiding guild does a 10 man raid, it will be on an off-night when most of the guild is not on.
Guilds will break-up and new guilds will be born, but the same players raiding now will be raiding after WotLK is released. Maybe under a different banner, but it will be the same players nonetheless
The players that are raiding now will likely continue to raid in the expansion. Players not raiding now will continue not to raid. I know of VERY few guilds right now that do exclusivly 10 mans (Kara and ZA). A guild could easily gear up in Karazhan and through Badge loot enough to finish all of ZA. Even get good enough to finish the timed event Heck, that's what my guild did before we merged and starting doing 25 man content (minus the bear mounts)
The main reason players PUG Karazhan now is because they overgear the place (S2 gear, badge gear). These players do not raid 10 mans because they dislike 25 raiding guilds (at least IMO). I think they only do it because it is now so easy with all the changes in the past 6 months. At least on my server there are no guilds that farm ZA as a progression instance. T6 guilds do it for bear mounts mostly, and the average T5 guild does it on off-nights for something to do.
Failure is not an option. It comes bundled with the software.