Tedium allows you to switch Time Machine backup destinations easily.
Tedium can be downloaded from http://www.tediumapp.com/download/0.9.0. It is currently in an alpha state so expect bugs.
Tedium is free, open source software hosted at https://github.com/dustinrue/Tedium. If you wish to build Tedium yourself you can do so by cloning the Tedium code to your computer. If you don't have git installed you will need to install either the GitHub client for Mac at http://mac.github.com/ which install a GUI client as well as the command line client or if you simply want the command line client installed you can get it from http://git-scm.com/download. If you need additional help with using Git GitHub is a great resource. You will find GitHub's documentation at http://help.github.com/.
Once cloned, open the project file in Xcode and issue command+b to build the app or command+r to build and run it.