Originally Posted by Crimson13
Nope, update.zip only goes one way
Are you 100% certain of that?
The incremental updates do verify that you're coming from the proper version of Android before installing, but the full ROMs don't seem to care as long as the device ID matches.
I think it's at least worth trying since if it doesn't allow downgrading with a full ROM, it'll just give you an error and cancel the installation.