Upgrades are at least partially decided at the check-in.
I have been upgraded in BKK from C to F three times last year and invariably it happened after I asked if the flight was quite full or not. This reminded the lady at the check-in about their rule that if the flight is full, they have to upgrade people. They first take ROP Gold, ROP Silver, and then Star Gold. Every time they had to call a supervisor to ask permission.

Another time we checked in with a whole group in Yangoon to fly to Bangkok and everybody in the group was upgraded from Y to C in advance. So both systems apply.

I don’t really think the fare code matters. We have already been upgraded from a discounted V-fare to C and even twice to F from a Q-ticket that had been upgraded to C by the Gold upgrade. At any rate being friendly at the check-in will help because most of the times it seems that taking the initiative totally depends upon the lady at the check-in desk.