On initial VL5.9 standard install, I created a non-root user, and it was automatically assigned to 1000:1000 uid:gid. Later, I wanted to change the ID numbers to 1001:1001, but keep the names. I logged in as root to ensure that user 1000 wasn't logged in or their personal files weren't in use. Then, ran "usermod -u 1001 [user-name]" and "groupmod -g 1001 [group-name]" Then changed directory to /home/[user-name], and used "chown -R --from=:1000 :1001 ./*"
All my user's files and folders were alright. But, sometimes when I created new files/folders, the GID 1000 got assigned again. Seeing this, I ran "groups" as that user and got an error about 1000 being the user's group, but that number not being assigned. (I forget the exact wording). In /etc/group, the line for that user's group had the correct number assigned, but no user's listed. So I added the user's name to that line in the file, saved it, then opened the group shadow file, and saw a line for the relevant group, but there was an exclamation mark, too. Left this alone.
The username is okay, but the user's group number is a problem.