I ran into something like this not too long ago.
The first and most important command you are looking for is chown
You are going to want to change a lot of files at once so you will want to make that change recursive over the home directory it will look something like this:
[davdunc@mps1530 ~]$ chown -R davdunc:davdunc /home/davdunc
I am using davdunc as both the user name and group name here because I expect you have "user-private groups".