I've done that once, going OAK-DEN-IAD. My original flight was in the afternoon. I went to OAK early to standby for an earlier flight, got a seat on OAK-DEN and, for my DEN-IAD leg, asked for both standby status and being put on the upgrade list. When I got to DEN, both the standby and upgrade requests had cleared (~30 mins before departure), and I got a nice biz seat on the 777.

Edited to say: Of course, I went standby for an earlier flight, so my fall back position is my original flight. In your case, standing by for a flight departing after your original flight is risky if the 747 has a fairly full economy cabin...