For Amadeus Tkt Changer to work, the fare would need to be auto-quoted by Amadeus (or another system) to begin with - and most BA RTWs are built manually, similar to the way a tax quote is done for a redemption booking - a fare (any fare) is quoted for the itin booked and then the fare value(s) are replaced with the applicable xONEx fare. This means Amadeus can't read the fare rules when it comes to trying ATC - the TST (equiv. to Phase IV in Sabre, pricing record in Apollo) has been constructed 'by hand'.
Some simple RTWs would auto-price in Amadeus but it was often easier to just do it the 'long way' than to try and get the system quote to actually work...