This kind of change -- from int'l to dom 777 (and reverse) occurs quite frequently. When it happens, the seat map must be reviewed since there are significant differences. I'm told the system does this automatically, then it's reviewed by a CSR supervisor to ensure everything is worked out OK.

For example, a few months ago I was booked ORD-SFO on a 777, initially an int'l version with 8A reserved using a CR1. A few days before the flight, I checked the on-line seat map via a phantom booking, and saw that the seat map was unavailable. I knew something was up -- and sure enough -- the plane at the gate was a domestic version, and my 8A had been changed to a 2A when I checked in.