I don't know how reliable this works, but I did hear from a friend that this worked for them:
First they said they wanted to change from a hub 4 to a hub 3 which was refused by customer services as "we can't add used equipment back onto the network".
So they hung up and redialled, and asked if they could activate their new router and gave them the information from their old hub 3, disconnecting the hub 4 before hand, and this worked a short time afterwards. So apparently they can.
The return bag they received for the old router they shipped the hub 4 back in, as they only have 350MB service which the hub 3 does fine.