Original flight was HKG-SFO-EWR in V class.

I called the Premier Desk to see if I could return two days earlier.

The agent said there was no change fee for HKG flights (presumably, due to the civil unrest). However, there was a fare difference since the new flights were a higher fare class.

I asked which fare class the new flights were. She said 'V.'

I asked if she could see what my original fare class was. She said the original fare class was V.

"I don't understand. If my original fare class was V and the new fare class is V, why do I need to pay a higher fare?"

The agent got mad at that point, so I just bit my tongue and paid the $47.

My understanding was that you only needed to pay a fare difference when you're changing your ticket to a higher fare class. I called back to ask another agent and the second agent said it's possible to still have to pay a fare difference when you're changing a ticket and the new flight(s) are the same fare class. The agent said I was originally scheduled to leave on a Tuesday, and now I am leaving on a Sunday and "fares on the weekend will always be more expensive than fares on a weekday [even in the same fare class.]" I know there are multiple fares that are filed with the same fare class, but I don't know if that even factors into fare differences when you change your ticket. Maybe I'm confusing ticket changes with SDC, in which the fare class rule applies?

Does this sound right (that it's still possible to have to pay a fare difference when changing your ticket to the same fare class)?