There is a normally charge for extra boxes. However, the deal I negotiated with Virgin (while still with V6s) waived the cost and I kept the same deal when moving to 360 (only my hardware changed - mine was swapped completely and wasn't a software change).
Try calling Virgin early on and speak to retentions (thinking of leaving - options 1-4-4 on the phone IIRC) and ask to be connected to the 360 team. That's the route I've followed with 360 issues and it's usually been reltively painless
