Fiji will pose another physical challenge to Wales before they face twice title holders New Zealand on Sunday (6.10pm, S4C) in their final Pool match.
Dragons and Cross Keys back row Toby Faletau has suffered an ankle injury which has put an end to his tournament and he is replaced in the squad by Llandovery flanker Edward Siggery.
The changes in the front row see Glamorgan Wanderers hooker Ieuan Davies replace Rhys Williams and Tonmawr's Joe Rees start ahead of Rhodri Jones while there will be an all new back row. Vice captain Josh Navidi starts at openside flanker ahead of Rhys Jenkins, Morgan Allen comes in for the injured Faletau and James Thomas and James King swap positions, with Thomas now at blindside and King at lock.
Behind the scrum, Scarlets scrum-half Gareth Davies replaces the other squad vice-captain Rhys Downes, James Loxton starts in place of Adam Hughes and there is a new centre partnership with Ben John and Scott Williams replacing Beck and Owen Williams.
Ospreys academy / Bonymaen prop Will Taylor will earn his first Wales Under 20 cap if he comes off the bench.
Head coach Phil Davies said, "We have trained hard this week and hope to be a little more fluent against Fiji. The Samoan game was a typically hard game but the players gave 100% throughout the match which paid off with the match-winning try in the end. Matthew Jarvis' accurate kicking was key and kept us in the game.
"We view Fiji as a genuine step-up from Samoa, with their athleticism and off-load game very much a threat. They lost 44-11 to New Zealand but gave the New Zealanders some problems throughout and matched them physically for long periods.
"With five games in 16 days, strength in depth is key and this is a chance to give players the opportunity they deserve to challenge Fiji and show what they can do."
Wales Under 20 team v Fiji (Club Atletico Estudiantes, Parana):
Dan Fish; Kristian Phillips, Ben John, Scott Williams, James Loxton; Matthew Jarvis, Gareth Davies; Dan Watchurst (capt), Ieuan Davies, Joe Rees, Lloyd Peers, James King, James Thomas, Josh Navidi (vice-capt), Morgan Allen
Replacements: Rhys Williams, Will Taylor, Joel Galley, Rhys Jenkins, Rhys Downes, Steven Shingler, Ashley Beck