On their own reservations are not valid.

If you purchase a replacement pass you can use them with no issue.

A claim on your travel insurance may cover the cost of a replacement pass.

I asked Renfe information in Madrid. They did not know the answer but suggested I get on board with the seat reservation. They said the only penalty (if not allowable) would be to buy another ticket.

Be aware that you risk a heavy fine if you board a train without a valid ticket. 

I did as the Renfre clerk suggested. They checked the ticket (seat reservation) at the entrance to the track but not at all after. So far, in Spain, they never check on the train because all trains require seat reservations and they check those at the entrance to the track.