South Korea says its first astronaut will now be a female engineer, after Russian officials rejected the initial candidate over a breach of rules.
Ms Yi will spend several days at the space station
Yi So-yeon, 29, is to replace Ko San, 31, on a Russian flight to the International Space Station in April.
Russia's Federal Space Agency requested the change because Mr Ko broke training centre rules, a senior official said.
South Korean officials played down the breaches, which involved removing reading material from the centre.
On one occasion, Mr Ko sent a training book back to South Korea - something he said was by mistake.
On another, he received a book from a Russian colleague that he was not meant to read, said Lee Sang-mok of South Korea's Ministry of Education, Science and Technology.
Mr Ko was replaced at the request of Russia
"The Russians emphasised the importance of abiding by the rules, as even small mistakes can bring about grave consequences in space," Mr Lee said.
Mr Ko, who beat more than 36,000 other applicants to the position, has been training in Russia since last year.
He will now serve as the back-up astronaut when Ms Yi flies with two Russian cosmonauts to the International Space Station in early April.
Ms Yi will spend seven or eight days there, conducting a series of scientific tests.
The mission will make South Korea the sixth Asian country to put an astronaut in space.