As we say in this question, "The copyright holder is never beholden to the rules of the holder's own license grant". The copyright holder is not bound by his/her own licence.
Suppose that Alice creates a library, say
libalice, which she releases under, say, LGPLv2.1. Although s2c requires that modified versions be released under LGPLv2.1, Alice is not bound by that, as she is the rightsholder. She may release her library (and/or modified versions thereof) under any other licence she pleases, including proprietary ones, either in parallel with the LGPL version, or subsequent to it.
You may not do that with
libalice, because you are not the rightsholder. If you get hold of a copy, and modify it, you have no rights to propagate copies of that save as the licence on the original permits. If you choose to propagate it, either you have accepted the licence, and are therefore bound by its conditions, including s2c, or you are violating copyright and can be hauled into court and (inter alia) proscribed from further distribution.
If Alice accepts contributions to
libalice from, say, Bob, and furthermore she does not require Bob to sign a Copyright Transfer Agreement (or a Contributor Licensing Agreement which permits relicensing) then she has lost this power as the sole rightsholder. Now she, also, may not release the combined work under any terms other than LGPLv2.1 unless she gets the consent of all other rightsholders.