Deputy District Attorney Dan Fehderau said the charges were switched after "new and credible information came to light that required a re-evaluation of the roles," the San Jose Mercury News reported Wednesday.
Duc Tong, 18, of San Jose, was charged last week with the fatal Jan. 26 stabbing of Richard Phan, 22, after a fight broke out at a party where participants had been drinking for several hours. Police believe Phan may have been trying to break up the fight.
Duc Tong and his twin brother Anh were arrested the following day, but Anh was not charged.
On Wednesday, Anh Tong, who had remained in jail, was charged with murder with a knife enhancement. Charges against Duc Tong were reduced to two felony counts of being an accessory to the killing.
Fehderau said the charges were flipped after San Jose detectives continued their investigation.
"We have to follow where the evidence goes," the prosecutor said. "This is an example of police keeping an open mind, pursuing an investigation and wanting to get it right."
Anh Tong remained jailed without bond. Duc Tong is jailed in lieu of $200,000 bail.