I think save games are more compatible than most people think. You can edit most aspects, except for the map, and still play the save game just fine. You can edit any unit stat, add/change new units, buildings, traits, ancillaries and same games will still work.

As far as I know, the save game file contains characters info, territorial info, army composition & positions, and probably some other stuff like diplomacy. Kind of like the stuff in descr_strat.

So basically, no need to make a new campaign every time.