Brightman announced her trip to outer space in August, saying she planned to stay eight days at the ISS in 2015, RIA Novosti reported Saturday.
Roscosmos head Vladimir Popovkin said that plan could be off the table because the Russian space agency is considering extending to flight from eight days to one month.
In that case, he said, Brightman would have to give up her seat to a professional.
"If it's a monthly shift, then it will be staff cosmonauts and astronauts who will be performing some short-term scientific research," Popovkin said.
He did not say whether Brightman's trip would be rescheduled or who her replacement would be.
If she makes the trip, the soprano would be the world's eighth civilian space tourist, RIA Novosti reported.