Well… of course you could use any image processing /manipulation tool to invert the green channel… for example with Imagemagick… :
convert input -channel green -negate +channel output
( which may be the tiniest ??)
and also some normal map apps do have a convert or switch (green)/ invert (height) button… like jsut to name some examples:
https://boundingboxsoftware.com/materialize/
Or maybe this (windows only?):