You'll get lower signal strength. About 10-15% lower if the 1000.4 is properly tweaked. I've already tried it.
But for some you may have no strength loss,using a dish 500, or even pick up strength, again depending on how the 1000.4 is tweaked.
The logical thing to do is to slightly repoint the 1000.4 to maximize strength for 61.5 and 72, and forget about trying to get 77. Cover the outside lnb with foil to help tune in the other 2 birds.