The government plans to send Yi So-yeon for the nation's first astronaut space trip, instead of Ko San, at the request of Russian evaluators.
The Ministry of Education, Science and Technology has decided to change the primary astronaut, Ko, as he was found to have violated training protocol twice.
Sources said Ko mistakenly sent a mission training manual home along with his personal belongings last September, which was sent back immediately.
Ko also violated the rule last month by acquiring a spacecraft pilot's instructions that he was not authorized to look at.
Russian side has urged the astronaut candidate to strictly follow the rule as minor violations could lead to serious consequences at times.
Ko will not be penalized, however, and remain as backup astronaut. Yi, 29, is a doctoral candidate in bio systems engineering, has been training along with her male counterpart after she was named the backup.