migrate the existing AWS PCA Root and Intermediate CA to another account

I thought that just as public ACM certificates cannot be transferred to another account, private CAs cannot be transferred.
https://repost.aws/knowledge-center/acm-export-certificate

So how about sharing with another account instead of migrating?
https://repost.aws/knowledge-center/acm-share-pca-with-another-account

You could share the PCA, but this terraform aws provider bug breaks issuing certificates with shared PCAs: https://github.com/hashicorp/terraform-provider-aws/issues/26868. There is a PR with a proposed fix: https://github.com/hashicorp/terraform-provider-aws/pull/39952