You will be prompted to re-activate Windows after the swap because it won't recognize the system anymore. Also, you'll need the new drivers for the chipset and so on at the ready.
Worst case scenario - reinstallation. But even so, you might get away with only a repair-install of the OS (keeping your programs, but repairing the OS files and folders, registry, etc).
I did this before with no need for complete reinstall. YMMV.
So no, you won't HAVE to reinstall, unless everything else fails.