Equestrian eventing's Sarah Cutteridge has been replaced with reserve Mary King by the British Olympic Association after her horse suffered an injury.
The BOA has approached organisers about the swap and foresees no problems.
"We are disappointed for Sarah and the owners of her horse, The Wexford Lady," said team leader Will Connell.
"However, we look forward to welcoming Mary. She and King Solomon are already in Athens and are preparing to compete in the three-day event competition."
The combination finished fifth at the European Championships in 2003 and fourth in their category at Badminton that year.
King has prior Olympic experience, having ridden Star Appeal at the 2000 Games in Sydney.