Amazon RDS Promote Db Read Replica to RDS Db Instance
When I promote a read replica to a db instance, does it replace the main db instance from which it was a read replica or does it create an entirely new Db Instance?


It does neither of those things.&sup1;

Promoting an RDS replica converts the replica, in place, into a new and independent master.  The action leaves both the master and the replica instances running, exactly as they were, except that the replication connection between the two servers is permanently severed, and the former replica becomes an independent, writable master.  

It cannot be rejoined to its former master as an RDS replica again after promotion, and it contains a copy of the data as it existed when it was promoted.



&sup1; This isn't true for Aurora, but the phrasing of the question implies that you aren't using Aurora.