i suggest your daughter send them a letter explaining the situation, saying that they are welcome to attend HER wedding, but that it will go according to HER plans, not theirs, and if they don't like it then they must feel free to decline the INVITATION.
as has been said, they are guests, nothing more, they have no rights to make any kind of demands whatsoever.
also mention that if a piece of jewellery that is being worn by an adult who is entitled to do so, if more important than their granddaughters big day, then they are not the loving grandpaarents she thought they were and they are not welcome anyway.
i am gobsmacked at the cheek of these people, that they think its their place to make these petty and pointless demands! who the hell do they think they are.