Well I would think that the fare rules in force at purchase....so just future purchases.

Now there have been reports on FT this week of UA trying to get the $150 change fee on past purchases when the change fee was only $100--but they balked when the customer forced them to look up the change fee at time of purchase. Now whether that was negligence or ignorance on the part of the agent, who knows. Now I suppose that if one didn't know the difference and didn't challenge it, they might get away with it.