Migrating from MySQL RDS to Aurora RDS with CNAME record
As far as i know you can not replicate Aurora (slave ) from EC2 (master), please correct me if I am wrong. Due to the super grant lack required to do the "Change Master". I would recommend to use DMS (databawse migration server) for that task.

migration steps:


set route53 TTL to 1 min
set read_only in current master, to make sure no more reads go into the master.
wait for all transactions go into the new master.
Stop DMS replication
change CNAME
...


Pay special atention if your application uses sticky connections, if so you can stop the old master completely to make sure all traffic goes into the new one.