Taking a PVE guild to the expansion

So the endgame guild I'm a part of (shout out to Gothic Bunnies on Thunderhorn-H!) is doing really well lately. We just got a nice influx of new members right around the time we started getting serious about MC and ZG, and just this last weekend, we finally made it to Domo (Rag, you're on notice) and, later the same day, downed Hakkar (that was an experience, let me tell you). So we're rolling right along-- we're already getting set for Onyxia, and by the time the expansion hits, we'll be ready for BWL.

Oh, right-- the expansion. I'm not in a position of leadership in this guild (or any guild), but you have to think the sudden influx of great items, new instances, and ten more levels (I just kind of assume everyone in the guild plays WoW enough to buy it as early as possible) might throw things off a bit. What's the point in farming MC for tier 1 stuff when even quest rewards in the expansion look better? And how can you get a group of 20 together for AQ Ruins when everyone has to work up ten levels again, will be starting new alts with the new races, and LFGing (with the new system) for PVP and the new instances?

In short, how do you keep a "mid-endgame" guild together when all the cool new stuff hits?

Now, if you're in Naxx already and half the guild is dressed in Tier 3, I'm sure it's not that big a problem-- you'll probably all go to Outland and level up together. And if you haven't made it to 60 yet, you're probably not worried at all-- you haven't invested enough time in this raiding system to care too much. But it seems to me that things are going to be pretty scattered when the expansion hits, and it's coming right at a time when a guild like mine is hitting our stride on the old stuff. I do trust our leadership-- I'm sure that if anyone can strike a balance between leading the guild into the new content and staying busy on the "old" content, they can. But I expect it to be pretty tough. I don't think we're wasting our time on the "old" content-- even if my ZG gear will be outdated by the end of the year, downing Hakkar was an experience I'll never forget. But with all the new instances calling, along with the phat lewts and new encounters, will current "mid-endgame" guilds ever feel the need to go for C'thun? Or will AQ become the new Dire Maul-- an instance that has some cool stuff and fun encounters, but often gets overlooked on the way to endgame?