SPACER – change of status from rare to common. Absolutely everybody must know what a spacer is…
I'm sometimes unsure about how seriously to take your pronouncements, RTT. Obviously there are people who don't know what a
spacer is, so it will remain rare.
Further to birdy's contribution, the online
Historical Dictionary of Science Fiction identifies the word as having been used for "spaceman n.; esp. a civilian who travels or works in space" since 1937, and for a spaceship since 1939. Probably gender-neutrality wasn't a consideration in the early days. Just the genre's fondness for coining new words.
Despite these two senses, plus others in web design and surgery, I don't think the word is sufficiently well known for a change to be justified.