Yeah. So, Minecraft indeed!
I spent a few minutes over the weekend playing with Tekkit Lite and Feed-the-Beast. I haven't decided which modpack I think will work best for us, but it's on my mind and I figured I'd mention that I am evaluating them.
Whichever one we choose, it'll mean rebooting the world again. The Tekkit Classic stuff is indeed incompatible with Tekkit Lite and the various FTB mod packs. Much of the overpowered Equivalent Exchange alchemy stuff like condensors and collectors are gone from the newer version of that mod. The other mods have added lots of new items, and of course, there are the new features added by the core Minecraft 1.3.x and 1.4.x releases that have been ahead of Tekkit.
Also, whichever option we move to means giving up on Bukkit and any of the server-side mods that it provided. We haven't been using much of them lately anyway; so, that's not a huge loss.