It will probably work though you will wind up with duplicate entries for your chipset fuctions (system devices, etc.) most likey and will have many registry entries that are unecessary/redundant and problematic.
In the long run your system will be unstable and chances are you'll wind up reinstalling anyhow.
Replacing a video card, or memory, or drive is not a big deal, but ANY time I replace a mobo/proc I plan on a clean install.
Besides with a new mobo combo you'll want a fresh install so you can really feel the improvement from your old.