How big of an increase in size from a zone 1 to a zone 2 dish??? And did he change the mount along with the dish, or use the old one?
If it's substantial size increase, and he didn't change the mount (don't know the differences between the two zone dishes) then the dish could be drooping due to the extra weight on the zone 1 mount. (if they are different mounts)
If he changed the mount then there could be a misalignment.
Usually for every 3 inches of diameter of dish size increase will add about 2 to 3 dB of signal increase to the LNB, this would not cause a reciver to be overloaded, as most have an AGC circuit to keep this from happening.
To test the dish to see if it has drooped you can use a broom sick or something else that can reach the bottom of the dish an give it a little push up, you would need someone to watch the "S" meter on the receiver to see if this helps.
Most of the time the mount and dish will droop due to gravity winning the pulling contest, a very small push up at the bottom of the dish will tell.