The is a way to edit the Prime 8 file, exchange one of the files in there and resave the file. Its possible that the file afterwards can be opened with Prime 7.
I always use this method to open P7 and P8 files posted here in the forum with my P6. But there are also files which after editing still can't be opened. Especially when they use features not available in the older version (like drop downs). In the past @LucMeekes often succeeded in converting these files as well by using a more sophisticated method.
The method I use is described here:
https://community.ptc.com/t5/Mathcad/Prime-backwards-compatibility-solved-Or-at-least-hacked/m-p/637537