You'd have to re-set GPxPatch (not a huge bother) and GP3Edit (just load up the required set and pop it into the game. Anything else, you can pre-set as GPxPatch loadable files. I'd naturally play with the physics file. Depending on the realism you want, you might set up all the circuits for a specific year, too. So most of the work is in the preparation.
When it comes to saving, well, no problem there as you just use different save names. But it wont save the rest of the special data with it. You could end up with quite a mess if you forget to switch over.
Nico bites yer tyres!