This can be done with a slight hitch. You have to make sure the name of the 7835 is the same as the name of the 7825 therefore this 2 systems can be on the same network at the same time. So put the backup file on a network drive somewhere and you should be cool. The other thing is you must do the restore on the same version of callmanager that you did the backup. That means if you backup 3.1(3a) then you must do the restore on 3.1(3a).
Hope this helps,
-Mckee