Revit Error - Can't open files saved prior to 8.1

[alsmail2]

I am getting an error message (" file cannot be opened because it was saved in a version prior to 8.1") when trying to open all my Revit files which were created in version Revit LT 2019. I recently installed Revit LT 2023. Initially I was able to open and upgrade models but this doesn't seem possible any longer. I tried downloading the oldest available version(Revit LT 2020) but I'm getting the same error message. At present I can't access any of my files outside of the select few that I had already upgraded. Is there a way of solving this issue?

[RPTHOMAS108]

To resolve this I suggest you open a support case with Autodesk directly via your account (if that is an option). I had a positive experience with that process in previous years. You will not get it resolved here because this forum is not for product related technical support questions.

If you say they were saved last in 2019 but now they can't be opened in 2020 then there seems on the face of it to be a valid support case for subscription; 2019 is now more than four versions back but 2020 isn't. Not sure how they define that i.e. does it count as 2020 not being able to upgrade and so still supported or the 2019 file format being out of support.