I think it's already happening...

If you look on UA board's schedule changes thread right now, you'll see a slew of changes that have just occurred in flight number only. My JFK-LAX flight UA23 just became UA361, presumably because CO23 is already DUB-EWR. Someone else has reported that UA1/2 (ORD-HNL-ORD) just became UA200/201.

Looks like UA's numbers are the ones changing... ()