I tried to point out last year that you can't really upgrade a Creative Commons license, something a lot of CC licensors claimed to be doing at the time, unless the new license you're after extends the same rights guaranteed in the original license. You can license new work under a new license, but you can't retroactively limit uses of your work for which you've explicitly granted permission.
In the comments of Lawrence Lessig's response to an interesting-for-its-silliness Reuters article on CC, Karl-Friedrich Lenz asks whether it might be a good idea to provide creators with a limited term license, one that takes into account drastic changes in how an author perceives a work (e.g. an author's CC-licensed song is suddenly a hit and he wants his damn commercial rights back!). Lessig responds:
As they're written, Creative Commons licenses run as long as the copyright (which we all know is a "limited term"). So if you've given away commercial rights, anyone who has accepted the license before you revoke the offer continues to have rights.
This is, of course, the point I was trying to make. But a limited term license is an interesting idea.