A retail company started with a single auto-mode VPC that automatically created a /20 subnet in every Google Cloud region. As they expand globally and plan to add many custom subnets, peer with a partner VPC, and tightly control IP address allocation per region, the network team is concerned about the fixed 10.128.0.0/9 range auto mode uses and the risk of overlap with the partner. What is the recommended approach to gain full control of subnet CIDR planning while preserving existing workloads?