Well, i'm not an expert, i'm not on PC and I restart a new playtrougth on every Patch/Hotfix, but I can provide some clues
-All the modifications (add in 1.2/1.21) works wihtout problems on a pre-1.2 save (like driving, craft, items cost,...)
-Some bugs/problems normally fixed in 1.2/1.21 still here on pre-1.2 save (just browse the forum to realize it).
If you want to play and risk meeting one of these bugs because you really don't want to do it again, it's up to you (maybe you won't meet any, who know).
If you want to replay "safely" (there are still bugs/problems, for me nothing too serious), it's better to restart on "fresh" 1.21 save. Maybe to test a new lifepath.