I just did exactly the same a fortnight ago. The old router supplied for ADSL was a Hub 4 which does not support VDSL. I had been using third party routers but plugged the Hub 6 in as soon as it arrived. On switchover day the process took no more than 10-15 minutes - first the ADSL carrier disappeared, a little later when presumably he (or another chap) got the to cabinet the phone displayed 'no line', five minutes or so later once the phone line came back it automatically connected to the new fibre signal as if nothing had happened and all was fine - 39Mbs download here, which seeing I am nearly 1km from the cabinet I am very pleased with.
Not sure how long I shall leave the Hub 6 connected before I switch to my Billion router which also supports BT VDSL, guess it won't be long...
Not had thanks for that info.
As it happens plans changed today anyway. Head gasket on car went so it's fixed now, off to pick it up. Guess router will be installed tomorrow instead!
Hopefully that'll run smoothly. Been a pain working from home lately