Yes, all you have to do is alter the condition. Make this condition "Not Installed And PATCH" and it should check for updates at the end of the patch.
The SetupCompleteSuccess dialog box invokes the CheckForProductUpdates action only if "Not Installed"; is it just a matter of changing this condition, or are there more issues involved?
However, I doubt it will show the "Do you want to check for Updates?" checkbox on this dialog. If you want to show that checkbox as well (and you are using Developer or Devstudio) you will have to make a change or two in the Control or ControlEvent table (basically to add "And PATCH" to the condition of the checkbox control).