Originally Posted by tom tulpe

Thank you very much Heloisa!

It didn't work for me though (TXL-FRA-LUX, trying to change TXL-FRA from a 19:45 departure [as booked] to a 19:15 departure: Only J fare bucket was available, and so I was asked for EUR 450 or so to upfare from P to J).

Could it be that for a routing AAA - BBB - CCC, changing only the AAA-BBB segment to an earlier flight under the new rules isn't possible unless you also change the BBB-CCC segment?