Well, Depends, I would do a reformat switching from one brand to another (I reformatted when I got my new AMD mobo, was using a celeron before).
If i was switching cards.. from lets say, a gf2 to a gf3, then no, I would not reformat. Going from Nvidia to ATi, (or vise versa), I probally would have.
I rather not have to deal with the problem, and by the time I upgrade, I would have probally needed a reformat anyway. It only takes several hours for me.
Jus what I do.
-Jonathan