The Investigative Committee, the Russian agency in charge of probing serious crimes, said it had reclassified the crime as hooliganism, which carries a lesser sentence.
The crew of Greenpeace's Arctic Sunrise ship, who come from 18 countries including Britain, Australia, Spain and Russia, had been facing up to 15 years in a penal colony if found guilty of piracy.
The hooliganism charge -- which was also used against the Pussy Riot punks -- carries a maximum sentence of seven years in a penal colony.
Russia charged the crew of the Dutch-flagged ship with piracy early this month after they attempted to scale a state-owned oil platform in a protest against drilling in the Arctic.
They are being held in pre-trial detention until November 24 in the northern Russian region of Murmansk.
The case has sparked an international outcry with stars such as British actor Jude Law joining vigils outside Russian embassies.
The Netherlands has taken Russia to the world's maritime court in a bid to free the crew members, but Moscow said earlier Wednesday it would boycott the hearings before the International Tribunal for the Law of the Sea (ITLOS).
Greenpeace's Russian office said in a statement sent to AFP the activists "should be released immediately."
"The Arctic 30 are no more hooligans than they were pirates. This is still a wildly disproportionate charge that carries up to seven years in jail," said Greenpeace's Vladimir Chuprov.
