This is the standard IATA rule for doing a ticket change. (See for instance the IATA Ticketing Handbook, changes chapter.)

For a whole unflown ticket, the old ticket is exchanged for a new one. Essentially, the value of the old ticket is used as a credit towards a wholly new ticket issued according to the situation today. (There is an exception if the first fare component is not changed.)

For a partially flown ticket, the old ticket is reissued, and all historical fares (and fare rules), are taken from the date of original issue. (It needs to have been theoretically possible to have issued the new itinerary on the original issue date.)

RTW or xONEx fares are not "special" in this regard. If the airline treats a rule change as a "clarification" as you put it then they may be willing to use the new wording. But if it really is a rule change, then strictly speaking they should use the old rules for a reissue. Of course you may be able to get away with it because these fares have a large amount of manual process in them.