Moving from BT to Sky fibre will mean that you need to be migrated in the exchange from BT exchange equipment to Sky's LLU equipment, this migrates your telephony, but Sky LLU also delivers their ADSL, but presumably they disable that, for 'fibre' customers.
I believe that you are also moved to another port in the FTTC cabinet , it's not clear why they do this, moving from one position to another when the old and new FTTC ports are the same, but it is possible that if dates are no co-ordinated you could get the exchange work done, then the FTTC cab work done later as different staff do the exhange work to the fibre cab work, or it could be that Sky move telephony first, then move the FTTC later.
....if this happened it's possible that as you would be connected to Sky LLU (but still on a disabled 'BT' FTTC port) you could end up syncing up as Sky ADSL2+ rather than VDSL until the switch takes place in the fibre cab.
You could ask Sky if all the dates are aligned , but even if in most cases everthing happens on time, there are occasions when they don't,
