I finally have been able to get back to my angled walls kitbash for CWU, but I seem to have run into a problem.
I've rotated the 1-inch wall tops to make a 45-degree cut, but when I rotate it back it is noticeably less sharp (blurry looking) than the original.

Am I suddenly doing something wrong?
Or do I need to reload the GIMP program ?

I had that happen to me when using GIMP to convert Thoumonts tiles to anchor-less. I only noticed an issue after I rotated the piece I was working on. I realized (AFTER I'd done several hours of work on it) that I had forgotten to set the resolution to 300ppi (instead of the default 100ppi) when I imported the image at the very beginning. So, I had to go all the way back to the beginning. I wish there was a way to set 300ppi as the default in GIMP.

Is it possible that is what happened in your case? Generally, you can tell the resolution by looking at the title bar when you have the file open in GIMP. For a typical 8.5 x 11 image, the resolution should say 2550x3300.

Yep - that was the problem.
Thanks, Ninevah.