Changing from normal broadband to fibre broadband is a bit more involved, as the work needs to be done in the street cabinet not the exchange.
As its Openreach (A BT Group Company) who deal with both Sky and BT, they may not have staff available to do the work before that date, or maybe there is an extra card needed in the cabinet.
You can get updates from the UK based order team using this link Order Live Chat Team
Any advance payments made before activation, will be refunded on you next bill. You may also be able to claim compensation if your activation is delayed past the agreed date.