usermod will change permissions for the user’s files inside his home directory. files outside his home directory will have to be changed manually.
man usermod lists the following caveats:
You must make certain that the named user is not executing any
processes when this command is being executed if the user’s numerical
user ID, the user’s name, or the user’s home directory is being
changed. usermod checks this on Linux, but only check if the user is
logged in according to utmp on other architectures.You must change the owner of any crontab files or at jobs manually.
You must make any changes involving NIS on the NIS server.