Thanks for all the responses, especially Sardtok, who seems to have experienced this exactly as I did. Which is to say: today the Texture Room changes are now updating themselves just fine...as though there never had been a problem. [see above]

I've always felt that my copy of Carrara was an 'idiot child'. Not all copies of Carrara. Just the one on my machine. Some days it works, on others...well, the mental pathways just go away.