I don't know if there is a way to access them later, unfortunately. I do own the 2012, 2016 and 2020 editions on Steam and disc copies of 2004 and 2008; the latter has been unplayable as long as I've bought it, which was over ten years ago. TPM2004 worked off disc on my old Windows XP but I don't think I ever played it once I switched to a gaming laptop.

I think TPM2004 would make a great "historical" re-release. It has a complete campaign built in, some non-historical candidates for variety, should run great and fast, and feels distinct from any of the subsequent versions. I'd love to see a version of the 2008-16 game up as well but that's a lot harder to figure. I want to say use TPM2008's skeleton, make a few menu changes, and import candidates from later editions, but I'm sure on the back-end that wouldn't be possible; there's some kind of reason TPM2008's unique map features didn't carry forward, right?