It wouldn’t be possible for the files to automatically be converted. That would only happen if you open them and save them again.
Are you on Mac or Windows? Mac would always show INDD files to be the last version installed so simply doing a get info wouldn’t tell you what version they were saved as.
I'm on a Mac. And no, it shouldn't be possible for the files to be converted; however, that is how it is behaving. After uninstalling CC and restarting, so INDD 5.0 is the only application on my machine, the file icons and "Kind" has turned back to CS5, BUT these files will still not open saying that I need to upgrade to a more recent version of InDesign. Here is an example of a file created a year ago in CS5, and you can see it was last modified in October of last year. (My CC trial was last month). It will now not open. This is the case for every INDD on my computer (including files created several years ago that have not been touched in years):
Can you post one of the files somewhere? I’ll open it here and check the file history.
If it is indeed a CS5 file we can dig deeper.
Put it on Dropbox and post a link here.
Unless you did the trial a year ago, this file hasn't been modified since then. So this is a really weird symptom,
I'm assuming that the old version you have is CS5. Right?
I would check that your old InDesign CS5 is still working correctly, that it can make files and reopen them. It might need to be repaired or reintalled.
Thank you both for your help. After another restart, it "snapped out of it" and is now letting me open these file, including the file in the screen shot above. That is probably an unsatisfactory answer to anyone else who runs into this (hopefully they don't!). So after the trial, uninstall CC, restart three times. Got it. :-)
(And really, thank you for your offers to help - it was a rather unnerving morning not being able to open any files!).
Glad you got it sorted out.
Lesson to be learned: Always reboot.