To start with, yes, because 64 bit will actually use 4GB and 32 bit will only use 3.5Gb.
Secondly, the software is supposed to do exactly what you're asking, but I'm very skeptical of applications like this. My experience is that settings, drivers and profiles between OS's are generally not fully compatible and that a clean install of the OS, applications and drivers is the only way to get a trouble free system. I'd say you could try it, but be absolutely certain to back up any important files first.