I think it highly likely that you will be charged to modify your details - I believe it's something like 100€ for a name change.

It might well be better to accept that these tickets will cost you more to change than to buy new ones with the correct details ... and if you were to check in online and then need to change the details at the airport, I'm sure the charge imposed would be substantial and you risk being told that you will need to buy new tickets there and then which would be astronomical in price, assuming that there were still seats available for the flight.