A problem you might have is that your old box could be automatically de-activated in a few days time (I think the details I was sent said 5 days).
Another complication is that Virgin are telling customers that it is not possible to go back once you've taken a 360.
I don't work for Virgin, so this is just based on what I've experienced. I would suggest contacting customer support as soon as you can to ask. And mention up front that it's about the 360 box as it seems only a small number of staff are allowed to do anything with customer accounts where the customer has a 360 box.