I don't see why one couldn't save a project in 8.5,, open it in X3 save it and re open in 8.5.
you'll get a nag screen warning about opening in an older version but it should still work.
I still have 8.5 installed and if I get a chance I'll see what's up.
But really, you should take the time to come up to date. The new stuff is miles ahead once you just get the hang of it. We all did this. I stayed with 8.5 for a long time but would never go back now.
Example- Drag and drop an audio bass track to a midi track and bingo- a midi bass track. ( well not bingo,, but after 15 minutes or so of editing it works)
My list of features I now use would make returning to even x3 hard.