Nightly builds deviated from the old file format long time (several months) ago. See Post-v5 new features and development news - #71 by eelik.
The nightly builds should always be able to open old files - if not, it’s a serious bug. But they always write new information there when saving the file. Going back and forth isn’t a good option. I don’t understand what you mean by
What exactly you want to do, just go back to the stable version or what?