Brown Panther owner Michael Owen and trainer Tom Dascombe have surprisingly replaced Richard Kingscote with Kieren Fallon for the St Leger.
Kingscote was wearing Owen's blue silks when Brown Panther won the King George V Handicap at Royal Ascot in June.
"I have spoken to Michael at length and it was a tough decision, but on this occasion we've decided to go for a more experienced jockey," said Dascombe.
Brown Panther is a 10-1 shot for the race at Doncaster on Saturday.
The colt's victory at Royal Ascot famously reduced the Manchester United star to tears of joy in the winners' enclosure.
Kingscote partnered Brown Panther in his two races after Royal Ascot, both defeats, in the German Derby and behind fellow big-race hope Census at Newbury.
Dascombe added: "Richard has always done a good job, but you don't get too many chances to win a Classic and I decided that it was a decision that had to be made."
Six-time champion jockey Fallon will be taking part in the final Classic of the flat racing season for only the sixth time.
He has finished second twice, once, in 1989, riding Sapience, ironically a horse on which he had been replaced earlier that year.
A maximum of 11 runners will line up for the St Leger, with the Sir Michael Stoute-trained Sea Moon the red-hot 11-8 favourite.