linux_mgmt_change-uid.howto
Theory:
usermod -u <NEWUID> <LOGIN>
groupmod -g <NEWGID> <GROUP>
find / -user <OLDUID> -exec chown -h <NEWUID> {} \;
find / -group <OLDGID> -exec chgrp -h <NEWGID> {} \;
usermod -g <NEWGID> <LOGIN>
Example:
#in recovery mode
find / -uid 1000 -exec chown -h 501 '{}' \+
usermod -g 501 pmichalec