Glencadam Gold was ridden by Tommy Berry to win the Metropolitan Handicap at Randwick on Saturday.
Cassidy has won the Caulfield Cup on Might And Power (1997) and Diatribe (2000).
Nash Rawiller is the new rider for Southern Speed, winner of the race last year in the hands of Craig Williams.
Glen Boss, who rode Southern Speed in the Turnbull Stakes on Saturday, declined the mount in quest of the ride on Glencadam Gold.
Williams rides Dunaden.
Trainer Leon Macdonald was mystified by the lacklustre performance of Southern Speed in the Turnbull Stakes.
Rawiller won the 2004 Caulfield Cup aboard Elvstroem.
He was the leading rider of group one winners last season.
Southern Speed is at $17 in pre-post betting. Glencadam Gold is at $4.80.
• Julienas, a stablemate of Glencadam Gold with Gai Waterhouse, has been ruled out of the major cups due to a slight virus.
The Wyong Cup winner has been set for the Tattersalls Cup at Randwick on October 20.
