Originally Posted by
guv1976
Is the ticket for travel to or from the U.S.? If so, and if the ticket has already been reissued to reflect the flight change, this might be of interest to you:
"Can an airline increase the price of my ticket after I complete the booking process?
- After a ticket is fully purchased – with either money or points – and the transaction is completed, the airline is prohibited from increasing the price of the ticket or requiring the passenger to pay additional money unless the airline provided notice to the consumer of the potential for an increase in a government imposed tax or fee and obtained the consumer’s consent."
(Emphasis in original.)
Source:
https://www.transportation.gov/indiv.../buying-ticket
That won't help the OP. The airline is saying they made a mistake and are offering to put the booking back the way it was. It's not too dissimilar to the way airlines can cancel fares offered in error (the so called 'mistake' fares).
However, as I said in my post, if the OP pushes back I suspect it will be possible to avoid the extra cost.