It’s the provider you are joining you need to ask, if they initially picked a date that was too early for your liking, then if they cancelled that order and re-ordered for a date more to your liking , then that date is in the wholesale system that both the old and new providers use , so the date the new company picks is the date Openreach do the work, there is nothing for the losing provider to do, apart from when they get advised the work is complete they calculate the final bill.
If the new company have cancelled and re ordered , then you probably will get a new letter from BT with the new date, if you don’t get a new message, you need to check with the new company that they cancelled and reissued rather than some sort of billing date change which won’t change the physical date, just the date they start charging you from.
The final bill will generated after the changeover , any estimate before hand would just be that, an estimate ( after all the actual date the physical work is done may drift by a small amount, depending on Openreach ) but the fact that your line rental saver and presumably your broadband account don’t align , if the broadband is 25 Nov, and the LRS is end of Jan 22 , the final bill may be full of small adjustments, debits and credits.
If you lose service as a consequence of this date change , it’s your new provider that you would need to contact to sort it out, the losing provider has no control over the process unless you were being ‘slammed’ without your consent
