How to migrate an already running RDB instance from IPv4 to IPv6 to stop charging

I have a RDS DB instance running with public access, so resources outside the VPC can connect to my database.

Due to the billing policy, Amazon has begun charging for IPv4, because of that Amazon suggest me to migrate from IPv4 to IPv6 not to be charged.

To do so, I've tried to follow all this guides:

• https://repost.aws/knowledge-center/rds-ipv6
• https://docs.aws.amazon.com/vpc/latest/userguide/vpc-migrate-ipv6.html
• https://aws.amazon.com/blogs/database/ipv6-addressing-with-amazon-rds/

After modifying all VPC, subnets, route tables and security groups, when trying to modify the Network Type in DB instance by switching to dual-stack mode, I'm getting an error

"The modify request failed because it can lead the database to a public dual state, which is not currently supported. Details of the potential conflict follow. Public accessibility: It is active in the current database instance. Network type: DUAL. It is requested in the current modify request"

Seems that public accessibility is the root cause, but as I said, I need the database to be accesible outside the VPC. Any suggestions or walkaround to tackle this issue?

Thanks