Agreed that the assumption should not be made. In case it wasn't clear above the new route had been checked by me as pricing onto exactly the same 6/7 character fare code as the original itinerary.

Having established that I thought that having already bought the fare it meant I had the option to reroute on anything that was valid for that fare and only pay change fees. I didn't know that I would have to repurchase the fare, so that is a good thing to learn ^.

To cut a potentially long story short the TA have (either on their own or through negotiation with AC) moved me back to a T+ fare and acknowledged that they have learned a lot about the AC fare structure that they didn't previously know .