Sorry to hear of your frustrations on your earlier journey.

I'm sure you're aware of this, but AFAIK for all airline tickets, the rules about changes apply within the same booking class. I can only find fare rules for C domestic, not D, but I understand that D is a lower class than C. So, if you had a D-class ticket, and wanted to change to a flight that only had availability in C-class, you would have to pay the fare difference between D and C (on a half round-trip basis). You would not be paying a 'penalty' (as you would have to if you were changing a cheaper, non-fully-flexible economy ticket) but you would still have to pay the fare upgrade.

Obviously, I don't know whether D was available on the flight you were previously trying to change to (in which case, it seems you should have been able to change free of charge), or whether only C was available (in which case , the request to pay the difference between D and C would presumably have been legitimate).

I would agree it is confusing that this isn't stated explicitly in the fare rules, but I guess it's a hang-over from the days when all bookings were made by travel agents, and it was assumed that all travel agents knew these things. If this was indeed the problem, it is also unfortunate that no-one was able to clearly explain it that way at the time.