So hopefully I can partly answer this question. Just be forewarned that I am not a professional historian, and my "expertise" is Korean politics from the late 16th to early 19th centuries, which doesn't necessarily concern concubinage. My answer here will focus only on royal concubines in the later era of the Joseon Dynasty of Korea. No discussion of concubines of nobles (Yangban), or any countries outside Korea.

So, were there ex-concubines of kings (that is, concubines who had their status removed)? Yes-in fact, it was not infrequent. For instance, Sukkung Gungju (a concubine of King Taejong) was expelled from the court when her father was punished. Gwiin Kim, a concubine of King Sukjong in the late 17th century, was also stripped of her titles and expelled from the court. The reason for this one has to do with the political conflicts at the time (not really sure you'd be interested, but I can explain in detail), but I don't think the two had a particularly good relationship in the first place.

What happened to the concubines that weren't "used"?

Even older concubines often remained in the court. Yeongbin Yi, for example, was born in 1696, and was a beloved concubine of King Yeongjo (she had six children). She continued living in the court long after she was no longer able to give birth; she died in 1764, in the court, and her death saddened Yeongjo greatly. Even Jang Huibin, who was disliked by Sukjong, stayed in the court until she was executed.