As what I understand the KiCad files are based on text instead of “code” it might be possible to open Kicad Schematics/PCB files made in for example v8 or v9 in Kicad v7 or v6? Then, what text in the .kicad_sch + .kicad_pcb & .kicad_pro files set what version the files are made in?
KiCad can’t open files from future versions.
I understand it that KiCad current version (like V7) just assumes that everything it finds in files have to have meaning KiCad understand. So if there is something added later it simply will complain that it don’t understand file and not try to find if file without these information will also be understandable.
Ok, that’s a pity. My apologies for mentioning this again but with the long gone Eagle this was possible (with some warnings when opening the newer files, but ok later when resaved in the older software version) and I think, not sure though, that Altium also supports this function so it might seem a bit strange it’s not possible in KiCad. Note that I’m all kind when saying this and I can’t explain once again why this is important so let’s end this topic here even if the case is “unsolved”, it’s no idea to explain programming details as it’s too difficult for me even if it’s text based project files 
Have a nice day!
You seem to not distinguish between a program and data file
Ops, yes I typed it wrong! It’s better I guess if I correct this now, now it’s done!
You could try your luck editing the text.
If only the header changed in the format, it might work.
If they changed other things it will choke.
The format is pretty readable.
A bit too verbose, and too many damn brackets if you ask me, but it’s not something anyone will read often so my opinion is a matter of taste - Ie irrelevant.
S-Expressions are based on Lisp - which many claim stands for ‘Lost in a sea of Parentheses’….
Whilst it might occasionally be useful to open a newer design in an older version of the software, important new features may not be supported so production from or even editing a new design using a legacy KiCad version might end up with an incorrectly manufactured board or some complex error / conflict issue. You may be able to fudge the issue by editing the header (I have done this when playing around with nightlies) but I’m unconvinced of the utility. You can run multiple versions alongside each other on most ?all platforms.
I would rather the developers concentrated on bug fixing and new features than partly supporting opening new designs in legacy software.
Thanks, I tried to change the header (version info) and it did not work. It complained about many error lines in the long, long text files (schema + pcb-file) , so this was not an option for me in this case.
I did a rollback on a few V8 files to V7, and it was a big job.
Editing the heading is not enough, you need to do search/delete for lines containing new keywords (eg, italic) and keep going until you run out of errors.
Tedious without end.
Yes, I recognised that too and gave up fast Maybe someone will make a thrilling plugin for this?
An old FAQ article…
The only possibility in the future would be for the later version to have an export rollback mode that strips out new features with dire warnings of what has been lost. Only this later version can be aware of what the new features to be stripped are.
Going back one version would be a lot of work for the developers, more than one version exponentially worse and potentially far more damage done to the project
The real question is why?
What realistic use case is this?
I can see (maybe) reverting back one version if the newest version has serious bugs, but in that case I would think that not much work would have been done using it.
Why would anyone want to open a version 9 file in version 6?
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Someone designing with a development version, not taking heed of the warnings, and believing rescuing is an entitlement. That user would probably consider themselves as a realistic use case. 
Then there is the "I want a circuit published in Kicad 8, but I’m only using Kicad 6 because I, don’t want/can’t be bothered/can’t afford, to upgrade my OS and I, can’t be bothered/don’t know how, to redraw the circuit in my current Kicad version.
How’s that for a couple of good quality, head banging, reasons to employ an extreme amount of time and effort by many, for virtually no return. 
This thread shouldn’t exist. The subject has been done to death years ago on this forum. It is time everyone stopped wasting time on this.
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