Fundamentally, I guess for UA, sure, they can say they're "moving" the flights to EWR. The capacity in November might be A+B, if A is EWR's transcon traffic and B is PS today. In a year its going to be A. People used to flying to JFK are just going to switch carriers, and people who weren't schlepping to EWR aren't going to suddenly do so.
They're not going to be attracting anyone new with this move, its not like they're starting up VNY-TEB.