I'm a bit unsure how generalized your question is.
Semantically, "reverse reappropriation" would just be called "appropriation". Double negations can simply be removed.
However, I don't think this is a case of "reverse reappropriation" in the first place, thus making the answer of "appropriation" equally incorrect.
Note that when I talk about "appropriation" in the rest of the answer, I am specifically talking about "claiming a word", not the general definition of the word "appropriation".
Appropriation is the act of taking something for yourself. Reappropriation the the act of reclaiming an appropriation. In both cases, the group shifts the word to themself.
But the semantical evolution of "social justice warrior" is not caused by those who call themselves social justice warriors. And therefore, "appropriation" does not apply here, in my opinion.
It seems to me that the definition you're looking for hinges on the cause of the semantical evolution, rather than the act of semantically evolving a word (which happens naturally and is not always a conscious decision).
And that is much harder to pin down and find an accurate answer for. You're basically asking "why do words evolve?", and that is a massive topic to discuss, that cannot really be contained in a single word or description.
So I will limit my answer to the following question:
Why do words sometimes evolve in a way that they receive an implicit negative connotation over time? What is it called?
This has been the case for "social justice warrior", and therefore seems the most appropriate (and possible) to answer.
There is a lengthy elaboration below, but it ends with me offering stereotype as the correct answer.
Social justice warrior
First off, let me discuss social justice warrior specifically.
Original: Someone who tries to enact social justice.
Nowadays: Someone who tries to enact social justice, using a wrong approach (most notably through logical fallacies in the interest of creating pseudo-intellectual rhetoric, and using facile arguments that prevent a meaningful discussion by shaming or silencing the opposing party)
Why did it evolve?
Because a sufficient amount of people who call themselves social justice warriors have been guilty of these transgressions. If a sufficient amount of social justice warriors are perceived to commit this transgression (from the subjective point of view of a sufficiently large part of the population), then the stereotype is formed where all social justice warriors are expected to behave this way.
If I had to summarize the cause of this semantical evolution, I would call it stereotyping. Because that's the reason for its evolution. The evolved meaning accounts for the stereotype that has been commonly accepted, at the very least up to a point of being commonly acknowledged (even if not everyone thinks it is factually correct).
In social psychology, a stereotype is any thought widely adopted about specific types of individuals or certain ways of behaving intended to represent the entire group of those individuals or behaviors as a whole. These thoughts or beliefs may or may not accurately reflect reality.
Wikipedia link. This focuses on the meaning of "stereotype" from a standpoint of social psychology, and delves into the different types of stereotype that commonly occur.
Since language and its evolution are strongly influenced (if not fully driven) by social interaction, the act of stereotyping can have a massive impact for evolving a language to account for "new" stereotypes.
This is normal human behavior. Humans are not logically perfect.
I ask millions of people to flip a coin 5 times. Statistically speaking, 1/32 people will get 5 heads in a row, and another 1/32 will get 5 tails in a row.
If I then ask everyone what they think of the coin, 1 in 16 people are liable to tell me that "this coin always lands on the same side". That is most evidently not true (it was a fair coin), but none of the test subjects are aware of the outcome of other people's coin flips. Their opinion is based on only their own experience.
If you do this experiment in the US, on a population of 300 million (give or take), you will end up with 20 million people who all erroneously believe this coin to be a fake coin.
Is their conclusion logically correct? Of course not. But it is the essence of human behavior: recognizing patterns based on a series of outcomes.
As much as we would like to paint them as wilfully evil, many people who partake in stereotyping (most notably racists and sexists) do so not because they are intentionally telling lies, but because their experience is vastly different from the statistical average, which leads to their opinion being similarly different from the average.
That is not an excuse for racism, sexism, or any other form of sterotyping. It is an explanation as to why it's statistically likely for outliers to exist, even in an unbiased system, given the human nature to base ourselves on what we experience.