IIRC, the only 'may not work' between P_12 and prior is anything that invokes Python due version change.

I seem to remember a sorta tutorial on how to edit 'old' python files to 'play nice' with new version...

Um, materials ? I recently loaded a 'classic' model, discovered each of the many materials invoked at least a dozen Superfly Cycles nodes. I made window wider and wider, more kept appearing. In the end, I just deleted all but their very basics. My 'post-middle-aged' eyes could not discern any quality loss...