In numerous re-issues and re-routes (on tickets issued by BA and AA) I have yet to have the taxes and fees decrease on re-issue.
You can get an idea of fees by entering the segment and carrier into ITA and just ignore the base fare. For example, QR, SIN-xDOH-JNB:
Code:
Fare 1: Carrier QR LHOWSG SIN to JNB (rules)
Passenger type ADT, ONE-WAY-ONLY fare, booking code L
Covers SIN-DOH (Economy), DOH-JNB (Economy)
$708.56
QR YQ surcharge (YQ) $129.00
Qatar Passenger Service Charge (PZ) $0.50
Singapore Passenger Service Charge (SG) $15.60
Singapore Aviation Levy (OP) $4.80
Singapore Passenger Security Service Charge (OO) $6.30
Subtotal per passenger $865.20
Number of passengers x1
Total airfare & taxes $865.20