Originally Posted by brunos

You have been booked in D with a fare type D*******. Each fare type (the ******) can have different conditions for changes.
If D is still available, you will be rebooked in D with the same type and have to pay a change fee (if any) and possibly a fare difference, if positive.
But if D is not available anymore, they can reissue the ticket in a higher fare class, usually leading to a stiff repricing.