A company operates a large-scale workload with numerous Amazon EC2 instances within a VPC, which includes both public and private subnets. The public subnets are currently configured with a route to an internet gateway for IPv4 traffic (0.0.0.0/0), while the private subnets route IPv4 traffic (0.0.0.0/0) to a NAT gateway. The company now plans to transition its EC2 instances to IPv6, ensuring that instances in private subnets remain inaccessible from the public internet. To achieve this IPv6 migration while adhering to the specified network accessibility requirements, what actions should the solutions architect take?