As David points out, the a license change like this isn't worth anything without the source code. Slapping a new license on something is trivial -- it's the process of releasing the source code that's also necessary.
I've heard of people setting up a dead-man's switch for other aspects of their digital lives. You could do the same here. Write a program that, if you don't check in for some period of time, will take the latest version of your source code, slap the GPL on it, and upload it to Sourceforge (or something like that).
You'd need to take some precautions, but no different from any other dead-man's switch. You'd need to make sure it's on a machine that will still be running after your death (e.g., not your home PC, if you think your family or the police will clean out your home, and if it's on a paid server that it's paid through your death + enough extra time). You'd need to make sure you don't accidentally trigger it early (e.g., if you go on vacation for a month!).
Alternatively, you could do it the old-fashioned way: a safety deposit box, or the like, with instructions for your lawyer/family to open it upon your death. It could include instructions to upload your source code to somewhere public. The upside would be that a lawyer might be more reliable than something you build yourself (e.g., they might have a good standard procedure for identifying when a client has died, and also know not to dispose of anything in that case). The downside is that then you'd need to physically transfer the latest copy of the source code to that safety deposit box regularly.