I made the mistake of scheduling an SNA-SJC flight on a Friday instead of Saturday as I intended. A call to reservations confirmed that I would need to pay the fare difference plus $100 change fee, if I wanted to change the date to Saturday. Alternately, she told me, there is NO PENALTY to standby for a different flight on the same day (Friday), which would work for me.

It should apply to both domestic and international, although I have had no experience with anything recently except full fare tickets.

I'm not sure I understand the upgrade question clearly. Are you saying you want to standby for a different flight in hopes of getting a better chance at upgrade? Well, you do realize that all upgrades will be processed ahead of standbys, and most operational upgrades are given to make room for standby passengers in the back. So unless the flight is not completely booked in all classes, your chances of upgrades are probably better on your original flight, since you can get on the waitlist and apply your status.