You can keep the old box and stream your recordings, thats what I have done, but it will cost you around an extra 11 quid a month... If you do do it, make sure that they will let you return your old box when you have finished with it. And also if you do plan to do it, make sure it is properly arranged before the engineer turns up.
Yes they did say it would cost £11 extra a month which I think is unfair based that it doesn't state that on any of the info on the v6. They should at least give you a month overlap free to give you chance to watch the recordings!