In the event if replacing an older STB with a newer one, the older one is removed from your account and should either be disposed of responsibly or returned if asked back for. The Scientific Altantic V+ STB is up for a free trade in for a TiVo and is not wanted back. Once you make that swap, the V+ is removed from your account, therein not usable anymore.
If you had however requested to have additional STB added to your account, you would be paying about £11.50 a month extra for the additional STB (TiVo) and your old V+ would of course continue to be active.