Hey there. The full game has quite a bit more content than act 1 and 2 combined. A lot of act 1 and 2 content has been re-written and edited, and the entire game is structured differently, so I would not recommend trying to transfer any saves. It'd be more enjoyable to experience it again as a new thing.
Steam used to give us devs the option to give discounts to people who already own another game, but they changed it shortly before we released the full game, so we could no longer do that. We wrote an apology post to our players at the time. We gave everyone a free fan pack, which has the OST and wallpapers, and some short stories from the same world.
So I hope this answers your questions and make you feel better about the situation, let us know what you think!