Mike Friday will play scrum-half and Niall Woods is back on the wing and will take over the goal-kicking duties from Chalmers.
The only change in the pack is in the back row where Rory Jenkins takes over from Scotsman Eric Peters, who is not registered.
If Quins win their remaining two pool games - the last is against Ebbw Vale on Saturday week - they are guaranteed a home tie in the quarter-finals.
Quins, meanwhile, have said they will not appoint a successor to head coach Zinzan Brooke, who resigned on 1 January, until the end of the season at the earliest.
Chief executive Mark Evans, who is standing in as coach, is drawing up a short list of potential candidates.
Supporters who buy two full-priced tickets for Harlequins' Tetley's Bitter Cup Final against Newcastle at Twickenham on 24 February will be offered the chance to buy tickets for England's autumn internationals against Australia, South Africa or Romania.