Rules for the North American (CONCACAF) regional championship tournament allow up to six changes after the group stage, which the Americans completed Saturday by beating Nicaragua 3-0, and US coach Bruce Arena took full advantage.
Former Fulham and Tottenham Hotspur forward Dempsey, 34, has 56 career goals in 134 caps for the US squad, one shy of Donovan's all-time record, while former Everton and Manchester United goalkeeper Howard, 38, has made 115 appearances for the US side.
Arena also added forward Jozy Altidore, midfielders Michael Bradley and Darlington Nagbe and goalkeeper Jesse Gonzalez, who makes his first US tournament appearance after being given a nationality transfer from Mexico by FIFA earlier this month.
Dropped from the 23-player lineup were goalkeepers Brad Guzan and Sean Johnson, midfielders Alejandro Bedoya, Cristian Roldan and Kelyn Rowe and forward Dom Dwyer.
"We have added some experienced players to the roster that can help us in the knockout round of the Gold Cup," Arena said. "The players leaving all made a good impression and I'm optimistic for their future with the national team program."
The Americans, who collected two wins and a draw in Group B matches, next play Wednesday at Philadelphia against an opponent yet to be determined.
Other than Gonzalez, the newcomers have all played for the Americans in World Cup qualifying last month with four of them boasting more than 100 caps.
The Americans are 2-2-2 in the six-team final round of CONCACAF qualifying for next year's World Cup in Russia. They own the third and final automatic berth on offer ahead of September's resumption of qualifying matches.
Arena replaced the fired Jurgen Klinsmann last year after the US squad lost to Mexico and Costa Rica, the teams ahead of them in the standings, in their opening qualifying matches.