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I don't think there would be a problem leaving the Ultra box in situ instead of a new box.
When I had (non multi-room) BT TV but used 2 BT supplied units in separate rooms I could switch my subscription from one to the other so I'd believe you'd be OK.
Most likely no, the reason being warranty, If BT supply you with a box and it goes wrong you'll get support, if you use the existing box and it goes wrong you'd not get a replacement but still be in a contract.
You don't have to use the new box but BT will supply one.
I have asked a moderator to provide assistance, they will post an invite on this thread.
They are the only BT employees on this forum, and are based in the UK.