Interesting.
Hasn't it been said that rules-sets can't be copyrighted? So if you just re-worded the self-same rules it would be "new",
legally. Some publishers might not care beyond that...
But the Law is an Ass, so that's not the true answer. I'd say, if it has changes that are
good enough, then it deserves to be called "new". In other words, it's totally subjective! (Sorry, perhaps that's not very helpful...)
'Derivative' is a loaded term, but not necessarily bad - I think anyone would have trouble coming up with good rules that weren't derived from (better to say "inspired by"?) some previous rules-set.