Not so many ppl started restarted their AP game since almost everyone playing this game already done at least 3 whole playthroughs before CW come out
2 new short campaigns are interesting to play but they are just that : short.
CW bring some new skills/items/spells/quests . . . But biggest change is completely remake of Orc race and how it effects old campaign; possible builds, tactics and unit combinations.
I dont know how far you got to current play in original AP, but remember it and after starting new game in CW you`ll see big difference.
As for save game files, if you download any mod for this game from this forum, you`ll have to start new game again and pick that mod to make changes. This doesnt mean for all mods, for example battle camera mod, or mana regeneration mod, silent digging mod and such will change already started game.
But any mod that change/add some unit, or object in game will have to start new game : all units on debir mod, adventure mod, red_sands mod . . .
So if you want to play again original campaign in CW before starting it check mod part of this forum and decide is there any you`ll like to try.