Hello,
how many KiCAD versions do I need to go back in order to save my schematic as a .sch file?
You will need to go back to v5.x
kicad_sch files were used from v6.0 onwards.
You can’t however open a kicad_sch file and save it as a .sch file.
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Were you hoping to share your schematic with a backward installation?
In general whatever the file suffix, one cannot go backwards for major version numbers, e.g. 8 to 7 or below, and going forwards is only one way.
As has already been written here, KiCad can’t convert projects to older KiCad versions. The closest you can get is import a bitmap (or svg maybe) of the schematic in an older version (if that older version can already import (and scale) images. and then place new symbols and wires over it. I have done this a few times, including with bitmaps made from pictures of hand drawn schematics, and it does speed up data entry.
But maybe we can some sort of better advice, but then you first have to tell us more about the background of what you are attempting to achieve.