Barrett was named in the side last month, but with the Hurricanes riddled with injuries he's been ruled out of the squad.
Blues first five eight Stephen Perofeta has also been ruled out due to an ankle injury.
Tana Umaga's nephew Thomas Umaga-Jensen and secondary school rep Ciarahn Matoe will join the squad for the Gold Coast tournament.
Under 20 head coach Craig Philpott said they've been following Umaga-Jensens return to rugby since his hip surgery in October.
"He's had three weeks of club rugby and is tracking well, we know what he is capable of after his time in the secondary schools team and he is a welcome addition now that he is injury-free."
He said Ciarahn Matoe will provide good cover at first-five-eight.
"Ciarahn's time with the Chiefs U20 squad has gone well and he proved himself with the New Zealand Secondary Schools team last year. This is a good opportunity for him and one I am sure he will make the most of.
"We had hoped that Stephen Perofeta would be available for selection. However, it's become clear he won't be ready in time for the tournament due to his ankle injury," Philpott said.
The under-20s play Fiji on April 28, Samoa on May 2nd and Australia on May 6th.