Just spoke to a very nice lady ad Zen, and there appears to be some misleading information about.
YOU DO NOT NEED TO MIGRATE YOUR LINE
I have just changed my Fibre Pro (200GB cab @ £45.60 per month) to unlimited Fibre 2 @ £35.60 per month No cap EVERY THING ELSE IS THE SAME, i.e username / password Fixed IP and T&C's
If you also want to migrate your line the costs change to
Unlimited Fibre 2 @ £30 per month
Line Rental @ 15.44 no option for discount for 12 month payment
or
Line Rental + unlimited calls i.e similar to the BT ( not know the exact spec) £ 22.60
Additional costs
Caller ID £0.90 per month
1571 voice mail £ 1.8 per month.
So for me I have a saving of £10 per month and got unlimited usage + I stayed with BT for my line
Well done ZEN