TEXT.img is the thumbnail in the editor to show the texture for BIT/LEV maps.
SOIL.img is the thumbnail to show destroyed soil texture of PNG maps and reside in Data/Custom/[texture] folders.
So i'm not sure why there is a SOIL.img in *some* of the Data/Level/[texture] folders, I don't think there needs to be (I might be wrong).
If you want to use these terrains with 3.6.31.0, I think your only option is to put the level.dir in an existing texture folder... and use the FileOverride module (
http://worms2d.info/WormKit)