Fedora 17, changed the default starting UID and GID for regular users from 500 to 1000. See the 1000System Accounts project page for more information.
So what is the best and easiest way to fix up existing files that might have lower UID and GIDs? It turns out that the GNU version of chmod has a very useful long option for doing exactly what we want.
Suppose you wish to change all files with a UID/GID of 500:500 to a new UID/GID of 1000:1000, here is how to do it:
chown -R --from=500:500 1000:1000 / chown -R --from=501:501 1001:1001 /
By the way, the system/user account boundary is set in /etc/login.defs.