Originally Posted by pandaperth

I have no direct experience of this. And my (maybe faulty) memory is that the fare rules in place at the time of purchase remain in effect for the entirety of the ticket. (IIRC this was in the context of an airline leaving Oneworld, but if you already had a flight booked with that airline, then that was still honoured.)

The barrack-room lawyer in me says that this must be the case - the contract between a pax and an airline essentially consists of the fare rules and the general conditions of carriage. For there to be certainty in the contract, these should remain on effect for the duration of the contracted service, in other for the duration of the ticket.
If a pax could request/demand the right to take advantage of a favourable rule change, then the airlines would in turn demand that unfavourable rule changes be enforced.