Obsolete QT files.. what to do?

Before I begin, I'm using a 2017, 21.5" iMac running Big Sur.

CPU: 2.3 GHz Dual-Core Intel Core i5.

RAM: 8GB (this is not upgradable).

Using Adobe Premiere Pro for my project.

I am trying to update my demo reel, but some of the sequences I want to drop into the reel are now deemed unusable. I just upgraded to Big Sur, and I am betting this has everything to do with it.

What can be done? Can anything be done? Do I need to move my Premiere project to Windows? Would that make a difference?

I have lots of older personal movie files as well. I understand Apple wants to update their software, but it seems that when they do, they like making everything earlier than, say 10 years, out of date and unusable. Well, I still want to use these files. I may want to use them till the day I die.

Help?