It would then allow httpd to write to the file.
httpd is allowed to write to httpd_squirrelmail_t files not to usr_t files.
So in case one day I decide to get rid of squirrelmail, how do I undo
You don't need to. If you get rid of squirrelmail (by removing the package), you'll have got rid of everything affected by this change.
Changing the context of files is similar to changing their permissions;
removing a file will remove any trace of changes you made to its