Italian lock Carlo Del Fava slots back into the starting line-up in favour of Ed O'Donoghue, who drops to the replacements bench.
David Pollock dons the number 7 jersey, while former Ireland flanker Kieron Dawson is rested for this match.
A knee injury has got the better of Clinton Schifcofske, who is replaced in the side by another new Southern Hemisphere signing, Timoci Nagusa.
Meanwhile, back rower Stephen Ferris will start after recovering fully from the rib injury he sustained against Stade.
In the backs, Cillian Willis has been preferred to Isaac Boss at scrum half, while Ulster-born talents Paddy Wallace and Darren Cave team up in midfield once again.
Commenting on the challenge his side will face against the Londoners, Ulster coach Matt Williams said: "If we play with the same intensity and effort we achieved on Saturday against Stade, then tomorrow's game will be very interesting.
"The boys are ready to have a good crack at 'Quins. We have nothing to lose and everything to gain, which is a good position to be in.
"Our physical commitment against Stade was great, but I would like to see the intensity of our overall game move up a notch or two against 'Quins.
"They are a dangerous counter-attacking team, particularly with pace on the wings. But we should be able to counter them and I honestly believe we will be there or thereabouts.
"Our biggest challenge is breaking this away game hoodoo. It's now up to the players in the Ulster squad to provide the enthusiasm and energy to produce the victory. You never know what will happen after that."
ULSTER (v Harlequins): Bryn Cunningham; Timoci Nagusa, Darren Cave, Paddy Wallace, Andrew Trimble; Niall O'Connor, Cillian Willis; Tom Court, Rory Best (capt), BJ Botha, Carlo Del Fava, Ryan Caldwell, Stephen Ferris, David Pollock, Robbie Diack.
Replacements: Nigel Brady, Justin Fitzpatrick, Ed O'Donoghue, Matt McCullough, Isaac Boss, Ian Humphreys, Rob Dewey.