JC – apart from the new term of “strata committee”, the only significant change to membership that’s relevant to your post is that building managers and property managers cannot be appointed unless they’re also owners.
So in the circumstances you describe where such a person is a member of the executive committee as a non-owner on the date that the new Act commences, then as I understand it that person could remain a member until the next Annual General Meeting unless, as is currently the case, a special resolution vote by the Owners Corporation at an earlier General Meeting removes them and declares their position vacant.