/usr/sbin/usermod -u 12345 joeuser
Now you have to change all joeuser's files to the new UID. Remember that ufs and ext* file systems store all ownerships with the UID, not the userid string.
/usr/bin/find / -user 701 -print | xargs -t chown joeuser
This will find all files with the ownership UID of 701 and reset to joeuser's new UID.