Backing up your EU3 IN or HttT is the only way to be able to play them. If you upgrade without backing up, you'll lose those original versions on your steam account permanantly. Any time you try to redownload EU3 after buying Divine Wind, you'll get the Divine Wind version. No way around that.
Reformatting your OS will delete those IN and HttT files, so back them up to a memory stick or something so you can alway have them.
I bought DW thinking I would be able to choose which version to play and found out at my expense that I cannot play HttT without rebuying it off of steam or on another account. It's rubbish, but Paradox say it's Steam's problem and Steam say it's Paradox at fault.
Reformatting your OS will delete those IN and HttT files, so back them up to a memory stick or something so you can alway have them.
I bought DW thinking I would be able to choose which version to play and found out at my expense that I cannot play HttT without rebuying it off of steam or on another account. It's rubbish, but Paradox say it's Steam's problem and Steam say it's Paradox at fault.