I'm in the process of doing the same thing. I haven't found a way (though that doesn't mean there isn't one), what I've been doing is changing all the reports to use a shared datasource. Right now they point to the original DB, but when I go to switch I should just have to change the details of the shared datasource and that should fix all of them at once.

i tried to do this with 2005 but the shared datasources never worked once I deployed. We're on 2008 R2 and that problem has gone away so It's time to fix it the way I wanted to originally.