[Russell Sobie]

I believe I read somewhere that Steam will allow you to delete a game from your library specifically for this purpose (to buy a package deal that is a better deal than buying just DLC). You could give that shot. Here's the instructions I found... I haven't tried this, but it might be worth a few bucks to you for the trouble (you'll probably also want to backup all your files before doing this, as it will likely delete the downloaded assets in the process):

Select "Help"

Select "Steam Support"

Select "Games, Software, etc."

Select the game you want to delete.

Select "I want to permanently remove this game from my account"

Select "Ok, remove the listed games from my account permanently"

Then after that, buy the full AC package.