I assume you're talking about the old Series 2 TiVos. Swapping the access card depends on if the receiver is a "RID" model or not (Receiver ID). The HDVR2 and similar models did not have a separate RID number to pair the card to the box, so the access card is transferable. DirecTV will no longer activate a non-RID TiVo as far as I know. The RID models require you to call in and have a CSR re-enter the RID for that particular access card.
There's some (possibly outdated) info here:
»www.tivocommunity.com/tivo-vb/sh ··· t=305017
They claim if you have a non-RID TiVo with an access card listed in the system as "owned", it will still work.
You can try to call in to DirecTV and follow the prompts as if you had a "not authorized" or "not activated" message. The automated system used to push out a "hit" to reactivate the cards on the account, but I'm not sure if it still does this or if it would work for a RID receiver.--
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