I'm trying to migrate my MySQL RDS to a different machine. The reason is that I need smaller disk, and you can't make disks smaller for RDS servers, only bigger.

I'm trying to migrate with this answer:
https://stackoverflow.com/questions/20638012/backup-mysql-amazon-rds#20645291
and this tutorial:
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/MySQL.Procedural.Importing.NonRDSRepl.html#MySQL.Procedural.Importing.Start.Repl

Briefly :

1. Created replica and stopped
2. Migrated from replica to a new server

3. Created user on Source DB with:

   CREATE USER 'repl_user'@'%' IDENTIFIED BY 'slavepass';

   GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%'

4. I've pointed to master from slave with this command:

   CALL mysql.rds_set_external_master ('source.mydomain.com', 3306,'repl_user', 'slavepass', 'mysql-bin-changelog.000001', 107, 0);

5. I've started replication with :

   CALL mysql.rds_start_replication;

When checking status with SHOW SLAVE STATUS, it shows that slave can't connect to master.
All servers are in the same security group. I was trying to connect to source with repl_user with MySQL command line and it worked.
I have no idea where to look next. I think it might be something with security in aws, but I was comparing new server with existing replicas and all seems the same.

I'm running MySQL 5.6 on both machines.