Frankie Dettori will replace Jamie Spencer as jockey on Toast Of New York after the popular racehorse was sold.
The Breeders' Cup Classic runner-up has been bought from owner Michael Buckley by Qatar's Al Shaqab Racing, for whom Dettori, 44, is retained jockey.
"It's obviously disappointing for Jamie, but Frankie is our man, and his experience in big races is second to none," said Al Shaqab's Harry Herbert.
Toast Of New York will still be trained by former jump jockey Jamie Osborne.
The horse, whose band of fans include American actress Bo Derek, was named after a 1937 Carey Grant movie.
His target for 2015 is the Dubai World Cup - the world's richest race at $10m (£6.6m) - on 28 March.
Bought for a relatively modest $60,000 (£40,000), his story has drawn comparisons with the rags-to-riches tale of legendary American racehorse Seabiscuit.
Toast Of New York's first victory came in front of about 500 spectators at Wolverhampton, but he has gone on to win more than £1.4m in prize money,
Spencer had announced his retirement at the end of last season before a U-turn and deciding to carry on.