Hello! I recently was wondering the same thing, as last time I upgraded my mobo and CPU, I was forced to do a clean reinstall of Windows 7.
Though just yesterday, I upgraded from an EVGA nforce 750i FTW mobo and Intel Core 2 Quad Core Q6600 CPU to an AMD Phenom II CPU and an Asus mobo. Despite even switching from Intel to AMD, my computer booted up successfully and loaded into Windows 7 without issue.
All I had to do was reinstall a few drivers, active Windows, and things are running perfectly now. I cannot speak for your specific configuration, but as I said, my past upgrade went without a hitch! Hope this helps!
Edit: And oh, the one time I did have to do a clean install of the OS (should that happen in your case), my current license key worked with my new configuration. In other words, I did not have to pay for a new license key due to the upgrade.
Actually, i plan on keeping my cpu. The Motherboard and ram are going to change. Ill be upgrading from a DDR2 ASUS M2n68-AM SE2 to a JetWay JPA76GM5D3-LF. The reason I dont't have to change my CPU is because my old board has support for AM3 CPU's even though it is a AM2+ socket. The whole reason why I am buying a new motherboard is because it is bottle necking the HT speed of my Phenom II X2 555 BE.