At number eight, Adam Balding gets the nod over Alfie To'oala who moves back to the bench.
Tighthead prop Paulica Ion returns to the run-on side after being out with a knee injury, and Michael Hills comes in at openside flank to replace Mike Denbee.
The match day squad also sees the return of the influential Tyson Keates and the international experience of Tom Voyce.
Harlequins boss Conor O'Shea has made six changes to the team that defeated London Irish in Big Game 5 at Twickenham Stadium last weekend.
Among the backs Ollie Lindsay-Hague comes in for Mike Brown at full-back, while Jordan Turner-Hall and George Lowe form the centre partnership in place of Tom Casson and Matt Hopper, with the latter named on the bench.
The final change in the back-line sees Sam Smith taking over from Ugo Monye on the left wing.
In the forwards, Charlie Matthews comes in for Olly Kohn in the second row while Tom Guest starts at blindside flanker in place of Maurie Fa'asavalu.
London Welsh: 15 Tom Arscott, 14 Dan Caprice, 13 Seb Jewell, 12 Sonny Parker, 11 Nick Scott, 10 Gordon Ross, 9 Rob Lewis, 8 Adam Balding, 7 Michael Hills, 6 Ed Jackson, 5 Kirill Kulemin, 4 Jonathon Mills (c), 3 Paulica Ion, 2 Neil Briggs, 1 Frank Montanella.
Replacements: 16 Greg Bateman, 17 Tom Bristow, 18 James Tideswell, 19 Matt Corker, 20 Alfie To'oala, 21 Tyson Keates, 22 Alex Davies, 23 Tom Voyce.
Harlequins: 15 Ollie Lindsay-Hague, 14 Tom Williams, 13 George Lowe, 12 Jordan Turner-Hall, 11 Sam Smith, 10 Nick Evans, 9 Danny Care, 8 Nick Easter, 7 Chris Robshaw (c), 6 Tom Guest, 5 George Robson, 4 Charlie Matthews, 3 James Johnston, 2 Joe Gray, 1 Joe Marler.
Replacements: 16 Rob Buchanan, 17 Mark Lambert, 18 Will Collier, 19 Sam Twomey, 20 Luke Wallace, 21 Karl Dickson, 22 Ben Botica, 23 Matt Hopper.
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