What is meant by "the existing Assembly" in this context?
In this context, "the existing Assembly" refers to the Legislative Assembly of a State (Arunachal Pradesh, Meghalaya, Mizoram, or Nagaland) that is in existence on the date when the Constitution (Fifty-seventh Amendment) Act, 1987 comes into force. This Act is being referred to as it is the one which brought the clause (3A) and clause (3B) mentioned in the given context.