No need to reformat... yet.
If you're going from nvidia to nvidia, no problem. Or ati to ati, no problem.
If you're going from one to the other, you have to remove all traces of the original drivers BEFORE you put the other card in. Nvidia's drivers are pretty coopperative (I'm told) and clean up after themselves. ATI's aren't, so you need to get their uninstaller (it is a stand-alone uninstaller). No matter which you do, be sure to browse through your registry to make sure that nothing is left there, as well as browsing your harddrive to make sure that nothing was left there either.
Reformatting should always be a last option.
My Rig: SABLE
Antec 300 Illusion / Antec EarthWatts EA650 650W / EVGA GTX 760 2GB
AMD FX 8320 x8 Black Edition / Gelid Tranquillo / MSI 970A-G43
Sandisk Ultra Plus 128GB / Samsung 840 120GB / WD Black 750GB / WD Green 1TB / WD Red 3TB
G.SKILL Ripjaws 2x2GB DDR3 1600 - Crucial Ballistix Tracer 2x2GB DDR3 1600
Win7 Ult 64-bit - Func MS3 Mouse - CM Storm QuickFire Rapid Mech Keyboard
R.I.P. Danny L. Trotter
... 14 Nov 1945 - 4 Sept 2009