Prominent rights lawyer Yu Wensheng has been charged with "inciting subversion of state power" and will be held in isolation far from his home.
Yu, 50, was charged with "disrupting a public service" after being detained as he left his Beijing apartment to walk his son to school on January 19.
But on Saturday police in Xuzhou, Jiangsu, weighed in with the subversion accusation. It corries a maximum prison term of 15 years - five times longer than the first charge, said his lawyer, Huang Hanzhong.
Police also told his family Yu will be under "residential surveillance at a designated place."
Known for suing the Beijing government over pollution, Yu has been a voice for reform despite the crackdown under President Xi Jinping. Hours before his detention he circulated a letter calling for five reforms, including the institution of multi-candidate presidential elections.
It is unclear why Xuzhou police are involved, but other rights lawyers' cases have been handled far from their homes.
Under "residential surveillance" rules in security cases, people can be held for up to six months incommunicado in unofficial jails.