Then BT will probably just take the line back (they did this with my old BE connection, and didn't use the MAC).
I had my line rental with Sky and broadband with BE.
It didn't affect my SkyTV. I just got a "sorry you're leaving us" letter from Sky (along with an "are you moving" letter - mainly because BT seemed to put my order through as a new occupier, at my home, who wants to take over the line from the old owner - no idea why, but that's just how BT work sometimes).
Unfortunately (although it has stopped all the PPI and home insulation scam calls) I ended up with a new phone number (and didn't want to risk asking BT to port my old number back in again - they screwed almost everything up with my order, so I didn't want to take the chance they'd mess up even more).
vDSL2 FTTC Infinity with BT
DL Sync 40Mbps
UL Sync 10Mbps