Hi @reimoosejaw,
Data resources do not require state migrations. They are read-only resources, so the action of reading them updates the state to the latest version. For this reason, Terraform does not call the UpgradeResourceState method for data sources.
I’m not sure what is going on the legacy SDK when using StateUpgraders on a data source, the error looks like it might be from json decoding? That may be something they can prevent in the schema validation.