Move a Terraform resource between two separate state files using state mv with the -state and -state-out flags

Steps

1. Back up both state files first (cp <SOURCE_STATE_PATH> <SOURCE_STATE_PATH>.bak and likewise for the destination), since state mv rewrites state files directly.
2. Identify the exact resource address in the source state, e.g. terraform state list -state=<SOURCE_STATE_PATH>.
3. Run terraform state mv -state=<SOURCE_STATE_PATH> -state-out=<DEST_STATE_PATH> <SOURCE_RESOURCE_ADDRESS> <DEST_RESOURCE_ADDRESS>, for example: terraform state mv -state=./project-a/terraform.tfstate -state-out=./project-b/terraform.tfstate aws_vpc.main aws_vpc.main.
4. Update the destination project's Terraform configuration to include a matching resource block of the same resource type/address so a subsequent plan doesn't try to recreate or destroy it, and remove the corresponding block from the source configuration.
5. Run terraform plan in both the source (resource block removed) and destination (resource block added) working directories to confirm a zero-diff outcome - no create or destroy actions - before applying anywhere.