Join Date: Aug 2011
Location: Sydney, Australia and Stockholm, Sweden
Programs: VA Platinum, SQ Gold, TK Gold, Hhonors Gold, Accor Diamond, Bonvoy Gold, Radisson Premium
Posts: 1,144
I'm not sure about your particular circumstances but different airports and countries have different taxes. I recall a redemption booking I made with TK for SYD/SIN/ARN had vastly different taxes to SYD/BKK/ARN.
My guess is your change resulted in additional taxes and they feel they should charge you for those.