Fallon had been expected to take the ride aboard Aidan O'Brien's 1000 Guineas winner Virginia Waters, but he will instead partner the daughter of Entrepreneur.
The Irishman rode Damson for the first time when steering her to victory against the colts in the Phoenix Stakes at the Curragh last summer and was also in the plate when she was third in the Cheveley Park Stakes.
The jockey's association with the filly has come about through his Coolmore retainer, as both horses are owned by Sue Magnier and Michael Tabor.
Virginia Waters will now be ridden by Johnny Murtagh.
Damson landed the Queen Mary at the Royal meeting 12 months ago but was ruled out of the Guineas at Newmarket after scoping dirty, while soft ground was against her when she was ninth in the Irish equivalent under Jamie Spencer.
"She seems in good form and hopefully the weather won't be too bad for her up here as she wants quick ground," said Wachman on his arrival in England from his County Tipperary base.
"I think and hope that she has come on for her first run, and she would appreciate true ground."
Meanwhile, Azamour and Ouija Board face a fascinating clash in the Prince Of Wales's Stakes tomorrow.
The pair are amongst eight horses who will go to post for the Group One showpiece following yesterday's final 48-hour declaration stage.
Azamour, trained by John Oxx, will be having his second start of the campaign having finished an unlucky-in-running fourth in the Tattersalls Gold Cup, while Ed Dunlop's filly will be having her first outing since landing the Breeders' Cup Filly & Mare Turf.
Norse Dancer, two and a half lengths behind Azamour last time out, will again take his chance, while Warrsan will bid to bounce back after his fourth placing in the Coronation Cup earlier this month.
Aidan O'Brien will be represented by Tattersalls Gold Cup third Ace with Australian raider Elvstroem bidding to go one better than when second in the Prix d'Ispahan.
Touch Of Land, who was fourth in that Group One, adds further international flavour when lining up for French-trainer Henri-Alex Pantall while Vodafone Diomed Stakes winner Hazyview completes the field.
Elsewhere, Antonius Pius is the only horse to have been supplemented for Saturday's Golden Jubilee Stakes at Royal Ascot at York.
It will be the first time Aidan O'Brien's colt has dropped down to six furlongs since last season's July Cup.
He has been running over a mile since and was last seen in the Lockinge Stakes in his only run this term.
The Group One event is the fourth leg of the Global Sprint Challenge, and 36 others stand their ground, including a number of those engaged in today's third leg, the King's Stand Stakes.
They include The Tatling, Var, Hong Kong challenger Cape Of Good Hope, American horse Mighty Beau and last year's Jubilee winner, Fayr Jag.
Others of interest are Resplendent Glory, who was an impressive winner of the Scurry Stakes last Saturday, and triple Group One-winning gelding Somnus.
Roger Charlton's improving filly Kind, Airwave from Aidan O'Brien's yard, former top juvenile Etlaala and Japanese raider Keeneland Swan are others to catch the eye.
Godolphin will wait until this morning before deciding whether Dubawi or Shamardal will represent them in this afternoon's St James's Palace Stakes.
The pair are among nine horses declared for the Group One race, but only one of them will go to post, with ground conditions on the Knavesmire the crucial factor.
Shamardal landed a Classic double for Godolphin, having also scored in the French 2000 Guineas, in which David Wachman's Indesatchel was flying at the death and went down by a head.
Indesatchel had notched up an impressive hat-trick prior to that, including wins in the Greenham and Tetrarch Stakes, with all those victories coming with ease in the ground.
"The faster ground is a slight concern, but I'd say it will be nice ground and he will take his chance whatever," the County Tipperary-based handler said today.
"He seems in good form and I hope he runs a good race. He has had a good season so far and the winner of the French race (Shamardal) is a very good horse.
"He showed in France that he gets a mile and we will try again tomorrow he seems to be improving with each race."
Clive Brittain's Kandidate holds Dubawi on their running in the 2000 Guineas after the pair finished third and fifth respectively, and the veteran handler expects the form to be confirmed.
"I can't see why it shouldn't be," said Brittain. He looks as though he is right back to his best and the fast ground certainly wouldn't be a threat to him, so I am going there with more than a hopeful chance."