[Edited 25/7]
Paolo, I have quite a collection of Logic versions; swapping 'backwards' between them has sometimes been a matter of necessity for me and rarely presented any serious problems - other than simply not being able to fulfil a more recent function (but that didn't usually bother me too much since I work chiefly in the Environment, which has hardly changed at all during the last 20 years).
(Oh and there's no need to pay any heed to the specious pest who mendaciously tries to smear and cry wolf at every opportunity; he's already been busted for doing that here and elsewhere).
Up until this new rule, opening a Logic project in an older Logic Pro version in some cases (as you mentioned) just flagged up a notification advising that the project was made in a later version; but generally it was ok. One big exception was the impossibility of opening a Logic Pro X project in Logic Pro 9 (in a MacOS capable of running Logic 9).
Logic was admirable in the old days (I have in mind LAP 3.5 F) for always making every effort to carry on manfully as best it could in the face of any sort of adversity; unlike Cubase which in those days tended to fall over at the slightest problem, often taking the computer down with it. (That was before Steinberg replaced huge chunks of Cubase's old code with pristine new code from Nuendo.) But nowadays Logic's behaviour is more like that of millennial 'script kiddies' - including some not as young but with similar gross lack of maturity, good sense, empathy, responsibility and intestinal fortitude.
Abnormality is the new normality. These 'script kiddies' in particular nowadays just do not seem to care - or even know - that they're incapable of reading anyone else's feelings. Their software code, and their posts in so-called 'social' media, reflect and ultimately betray their sociopathically narcissistic mentality. I wouldn't trust them to code a TV remote control device, let alone a DAW. Over the years I've known and worked with very many software professionals, many of them real adepts and a pleasure to work with. And I've had to deal with 'script kiddies'. I learnt to know the difference the long, hard way. These days however, it appears the proportion of adepts in that profession is alarmingly small.
So today we're already in, erm, "interesting times"; God only knows what lies ahead.
I have a happy dream that one day Apple will lose, big time, to a huge class-action lawsuit that calls out their fraudulent use of the word "Pro" in Logic Pro. Lolol.
Now I'd better shaddap and offer my apologies to VSL for veering so far off course from VSL topics.
