Do we need to ditch the existing BT Sport in order to add on Big Sport
No, if the existing BT Sport promotion is on the same account, the system will detect it and remove it during the order journey, so you won't pay twice when subscribing to Big Sport
Also, does Now TV cover Netflix and Amazon Prime?
No, Netflix and Amazon are separate companies, they have their own subscription, you can subscribe directly through the separate companies or via BT, there's no difference apart from the convenience of 1 bill - 1 direct debit.
And finally, can we give a starting date, e.g. the beginning of March for this to begin?
6 weeks is the furthest in advance you can request, it would mean leaving an open order on your account which would prevent other changes until the TV order closes down.