It shall not ruin anything. Old parts are converted automatically and shall look exactly the same before and after. If it is not the case, let me know.

For example I often used various strings without theirs endings - different balls and sticks. Now they have returned as inseparable part of strings and that's the issue.
And honestly for some tasks your new flex system is just too good, rendering nicely curved strings instead of stepped and angled ones.
That's would be incredible cool if you would be able to restore old flex system as back-up variant.
Sorry for being annoying.

I understand but this is not an option. Both systems have nothing in common. That is not possible to have both living under the same roof. As I understand this is more an issue of having hacked the old system by separating elements that were incorrectly designed as groups instead of inseparable assemblies.

If there is an existing bear version of the parts, I could do something in the converter to swap the part type for old system models. That would work.

Well, let's try another way. Is it possible to get access for older build of Mecabricks to download all the models in their pristine shape and forget about this?

Nope. But really it shall be very easy to do that in the converter. e.g. Replace part 14226 (String 11M W. 2 Knobs) that was only a string before by 56823 (String Poly 0.5mm on Spool) which is still the string alone. That is just an ID to swap. You would not see the difference in your model. I just have to make a small table to know what to swap with for older models.