If you really want to make sure that your AU has become (or can become) original and fully independent, this is what I suggest:
[Note: I'm trying to make this advice work for any type of story, not just yours, so I will mention things that make no sense in reference to RWBY.]
1. Break both stories into categories
a. theme and purpose (what is it about? what is the objective?)
b. physical setting (world, countries, biomes, ...)
c. temporal setting (contemporary, future, past, ...)
d. characters (main, secondary, groups and allegiances, protagonists, antagonists, ...)
e. main events and progression
f. tone
2. Compare both stories step by step
Keep in mind that, as many folks say, all stories have been told. While I personally find that a gross exaggeration, the fact is that, if one compares Tarzan and Mowgli, the starting point is much the same: orphaned child is taken in by animals and must defeat both animal and human antagonists. This means that your story can have the same premisse or starting point as another and still be original.
However, if, as you compare each category, you find yourself having too many simlarities, think it over.
If I may, I believe the most crucial categories to make your story truly original are the last three.
Analyse protagonists and antagonists on each side very carefully: what makes them tick, what their strengths and weaknesses are... and, above all, look for cosmetic differences. I actually suggest that you play your own devil's advocate and actively look for similarities. What else? If there is a group, check to see if it encompasses the same number of people and, if so, if they recreate the dynamics of the first story's group.
What about the antagonists? Are they after the same goal? Do they act similarly? I mean, if the original antagonists are demons trying to destroy humanity and you also have demons trying to destroy humanity, it's not necessarily bad. But there must be enough characteristics separating them: appearance, skills, MOs, origins...
If the main events and how they progress is similar, that's a red flag, too. I don't mean the general progression, though. The typical course of an action film is always the same, for example: Hero wins fight and is the greatest, meets foe and loses, fights again and loses again, the world as we know it is now on the edge of destruction but the hero finally saves it with a last minute sacrifice. No. What I mean is if the events that mark the protagonist's (and secondaries') personal progression mirror each other, happening at a similar time and with a similar result time and again.
Finally, tone: make sure both tales have different tones. Unless, of course, the tone is what you like the most about the 'original' story and you want to keep it. In that case, the other categories have better have fundamental differences.
Finally, a word of advice: use common sense.
One could imagine that Tarzan's author was so hung up on Mowgli that he decided to recreate the exact same story but simply gave it a different setting. The result, however, was an original story (even if with the same starting point).
