How many of your users are <= 1023?
You'll need to get new unused numbers. Then do a find -exec chown to change the ownership of all of the files owned by those users.
A good starting place would be the current home directories and search & change each. You may not care about searching elsewhere?
If you have only one user:
find / -name old-user-name -exec chown new-user-name {} +
If you have many, you should do the massive search once:
find / \( -name old-user-name1 -o -name old-user-name2 ... \) -exec ll -d {} + > file_list.out
Then write a script to do chown on each user's files in file_list.out
Do you use ACLs for these users?
