migrating advice

hi all , just about to request a mac code from BT and move to ADSL24 or UKFSN , few questions i hope someone can answer

i currently pay my ADSL and phone bill monthly as 1 combined payment by standing order , now , if i move to UKFSN do i call BT and tell them to cancel my direct debit and send me a new direct debit agreement just for my phone bill or do they amend it themselves once my new ISP has taken over my ADSL . and also once i request a mac code if i dont use it within 30 days do they cut you off ? thanks for any advice as im a bit wary of being billed for 2 ADSL connections as the whole point of moving is costs in the first place .lol