I dunno... I did a reissue last week in which I deleted 3 flights and added 9 new ones (originally a 14 seg e-ticket, now 20 paper) through AA. Due to a communications goofup I had to stay on the phone on hold with the RTW desk while the rates desk did the tax calcs in real time, during which the RTW rep came back on the line from time to time and advised me on how things were progressing.

At one point she said, "They're deducting the Canadian taxes you've been charged and replacing them with the new US taxes," [for some of the new segs] and also commented on how I was sliding on UK APT and they were crediting something re a FCO-MAD trip I wasn't going to be taking now.

So it sounded like they were indeed doing debits and credits on all the applicable old and new segments. That said, the final tally was that I owed them $100 for additional taxes and fees, which was an awfully round-sounding number.

Not that I'm complaining. The old route was NRT-LAX//YVR-JFK//SEA-DFW-ANC-DFW-SEA-ORD-FCO-MAD-JNB-HKG-SIN-HKG-CTS and the new one is NRT-LAX//SEA-DFW-ANC-DFW-SEA-ORD-LAX-xLHR-IST-xLHR-TLV-MAD-JNB//CPT-JNB-MRU-JNB-HKG-SIN-xHKG-NRT, so that's a lot of new T/Os and landings for a hundred bucks in new taxes.

(BTW, the $100 brought the total taxes/fees amount to US$512 for a DONE4, or roughly 8.4% of the ex-Japan ticket price. I'm sure I avoided a bunch of fuel fines that would have accrued if it hadn't been sold by AA.)