If it's the inbound, and in the same booking class, then effectively there should be no fare difference. Technically the fare on that segment is typically calculated based on the fares in effect at the time you purchased the fare, but there would need to be that particular language in the fare rules (which is typical). You did save the fare rules, right?

One wrinkle - to/from Asia, fares are almost always different for Mon-Thurs vs Fri-Sun (or similar). But probably not $600 different in coach.