I was able to get the new file properties to stick by removing and then reimporting the files. It took a while, but it is finished now (there are a lot of ISO's). To be extra sure it wasn't going to "remember" the previous files, I changed the name of an upper level folder that contains my ISO's.
But now, however, anytime I manually initiate an auto-import, it takes a considerable amount of time Even if I do it once, and then do it again immediately. Most of the time is spent with the message "Updating library to match external changes". It stays on "File 1 of 5291" for a few minutes, then eventually start to blast through them all pretty quickly. In the end the report always says "updated 5291 files that had external changes".
Shouldn't it be aware of these changes after the first time?