Just thought I'd mention this.
I pre-ordered the game and downloaded the telltale version. When it came out, I won two tf2 items and some other stuff. Here's how I kept them:
I opened Steam and downloaded the install off it, but I did not install it.
I just ran the telltale version through Steam and the two items I had showed up along with their achievements.
Any hand or win based achievements had to be re-won but I still had all my unlocks. Don't know if that helps anyone else, but it's worth a try.