Dettori came in for criticism from owner Bjorn Nielsen for his ride on Stradivarius in the Gold Cup at Ascot, where he finished third to Kyprios when attempting to win the race for a fourth time.
Now Nielsen has decided to replace Dettori for what could be the eight-year-old Stradivarius' final outing.
The John and Thady Gosden-trained chestnut has already won the Goodwood Cup four times, including under Atzeni in 2017 and 2018.
"Frankie Dettori, who has enjoyed a great relationship with Stradivarius and ridden him to many brilliant stakes victories, more than he has any other horse, will by mutual agreement give up the ride to Andrea Atzeni in the Goodwood Cup," Nielsen told Racing TV.
"Frankie remains not only 'the Strad's' biggest fan but a very good friend and of course I look forward to seeing him continue his phenomenal career as one of the greatest big-race jockeys of all time.
"Andrea will be no stranger to Stradivarius as they are undefeated together, winning the 2017 and 2018 Goodwood Cups as well as the 2017 Queen's Vase."
Thady Gosden added: "Andrea will be on him at Goodwood. He was on Stradivarius for his first Group victory, I think it was, in the Queen's Vase, so knows him well."
John Gosden reports Stradivarius to be in good heart ahead of his possible swansong.
He said: "Stradivarius is happy. He has maintained his enthusiasm for the game throughout and enjoys his training. We couldn't really be more pleased with him for a chap at this stage of his career, so we are very much looking forward to it."
The son of Sea The Stars boasts an unbeaten record at Goodwood, but his planned run last year was scuppered by soft ground.
Gosden added: "He was at Goodwood in the stables last year, when Thady and I walked the track. The water was lying on the top of the track, and we knew that wasn't for him. It is a pity he couldn't have run because every time he has run at Goodwood, he's won a Goodwood Cup."