I’ve just spotted that Semicharm’s free Poser script ReplaceTexture V1.1 had a May 2019 update. Now works with older versions of Poser.
Just a few easy in-situ clicks to replace a texture and all instances of it, with the option to shift Gamma too.
Not sure that it works with Poser 11, though. It doesn’t give errors in Poser 11, but it only lists one material on a selected character rather than the expected enormous list.