Use MCNostalgia to downgrade back to 1.2.5 minecraft, and then you could use Minecraft Portable so that you can have 1.3.2 and 1.2.5 in different folders and run them as you like!!
Once you have Minecraft 1.2.5 portable running in a new folder, you can go ahead and update your normal minecraft to 1.3.2 or whatever version and now you have both versions at your disposal!
1.3.2 removes single player as we know it, forcing all mods to now be mutliplayer compatible! While this may sound exciting, it will take awhile for mods to migrate to the new multiplayer format. So, grab the apps below and continue to game in version 1.2.5 until MoreCreeps and you other favorite mods are updated!
I will start working on upgrading MoreCreeps to 1.3.1 when Minecraft Forge is released, but until then I plan on updating the 1.2.5 version with more wacky stuff and the usual bug fixes.