If your account is provisioned for two V HD boxes and you replace one with TiVo, when you activate it one of the V+ boxes will be replaced on your account, leaving the other one.
Its pretty much irrelevent which one you replace with the TiVo.
The one you replace will no longer work. If you have not been asked to return it, might as well pop it away for if the other fails. They are getting on a bit now. If you need to bring the old one back on line, you will need to call in for them to reactivate it.