I have moved from DR to BT, in a nutshell BT will tell you they can not use the existing line as it is connected to BT. They will then offer to install a new line at a cost of £125. Stand your ground and tell them you would like Infinty (2) but are not prepared to pay. I did this, they said they would install the line free but continued to use my DR line when they arrived which just disconnected my DR service.
It means having a new telephone number but hey ho !
I have to say my Infinity 2 connection is far superior than my DR connection ever was.