The tax is charged to the airline, not to you. Any increase - and decrease - of the charge affects only the airline. They can change ticket prices accordingly but there is no obligation as far as I can see. Of course, airlines being airlines, most will raise ticket prices the second the charge goes up. But when it goes down, most will find a very good excuse not to lower ticket prices or reimburse anyone. So, don't count on it. It would be extremely unlikely.