The agency said in a statement that it made a decision to adjust its assignments "in part" due to "spacesuit availability at the station".
Since a second medium space suit torso will not be ready, Koch will wear the only available medium allowing her to go on the coming spacewalk.
Instead, Koch and Hague will make the spacewalk, which is scheduled for 7:20 a.m. Friday.
The spacewalk is still set for Friday, though the work of installing lithium-ion batteries on the station's solar panels will continue with NASA astronauts Nick Hague and Anne McClain switching places.
McClain now is tentatively scheduled to perform her next spacewalk the third in this series on Monday, April 8 with Canadian Space Agency astronaut David Saint-Jacques. They joined NASA in 2013, the year NASA's astronaut class was 50 percent female. More than 500 people have been to space, but less than 11% have been female.
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Nasa said McClain became "the 13th female spacewalker" earlier this month, and Koch will be the 14th later this week when she and Hague complete their spacewalk.
Koch, an electrical engineer, is taking her first space flight, according to NASA. The first female NASA spacewalker was Kathryn Sullivan, also in 1984.
NPR pointed out that this has been a longstanding issue that surfaced in 2006, when a report suggested women's opportunities within the space agency was limited somewhat by the sizing of spacesuits, which only came in medium, large and extra large.
She thought she could use a large one, but changed her mind after her first real spacewalk.
There have been 214 spacewalks on the International Space Station since 1998, but all spacewalk teams so far have comprised at least one man. Friday's spacewalk, to upgrade batteries on the space station, is expected to last around seven hours. Thankfully, Sullivan told Space.com, she'd only had one or two rude comments about her gender during her time as an astronaut, perhaps because "walking into NASA with the title astronaut would be a little like walking into the Navy with the title admiral instead of cadet". Among those factors were delays in the spacewalk caused by changes in crew assignments and schedules after the aborted Soyuz MS-10 launch last October.