Official mutations on left 4 dead 2

I just bought Left 4 Dead 2, and I noticed there's a "mutation" (mod) to play, but only one per week (I think it's per week).

Is there a way to play past official mutations, or is it intended for only one to be available at the same time?

It is now possible to play old mutations; when you select Mutation, the last option in the menu is "Change Mutation," and there's a pretty good list of Mutations you can play, in 1-, 4-, or 8-player games.

Note that, because there are a couple of dozen available mutations (counting all modes), it can be hard to get a game going unless you're willing to wait a while or invite some friends. If you want to play soon, invite friends or get used to bots. ("Four Swordsmen" is awful with bots, by the way. If you get grabbed by a Smoker on a ledge, your teammates can't figure out how to get to his ledge and stab him.)

If mutations work like dev-picks servers on Quake Live do, then no, there is no easy way to play past mutations.

Chances are, however, that are not-as-easy ways to play mutations. Often, you'll be able to regenerate the effect with existing cvars -- usually cheating-protected ones, however. For example last week's "dev pick servers" for QL was slow motion servers, aka timescale 0.5.