You can work with newer version files, yes, this has always been the case with Altium.
Quoting finance bros: "Past performance is not a guarantee of future results"
So far, Altium has always allowed you to
open files created in newer versions with older versions of the software, but they have not always allowed you to reliably
use them in the older version. Over the years, there have been many complaints that the program misbehaves in strange ways when working on files created in newer versions. This is the case even when none of the newer features are being used.
Even if you ignore those reports as user errors, the files can only work reliably as long as newer features are not used. Why are you paying support if you cannot use the new features?
With this new offer, they have strong incentives to make future file format incompatible so you can not go back to the old version without losing all the interim work.
Right now, there are only two viable options for staying with Altium:
1) Keep your perpetual license and accept that when your support expires, you will be stuck permanently with the then current version.
2) Switch to time based license and accept that you are stuck paying Altium every year, and paying them whatever they demand, otherwise you do not get access to your data.
Pick your poison. Some times, a good outcome is not on the table and all you can do is to minimize your down side. This is one of those situations
