It is currently possible to update workflows in bulk using POST /rest/api/3/workflows/update.

According to the documentation, though, this endpoint requires the workflow ID, version number, and version ID. The workflow ID is different on every site (even on sites copied with the all-in-one migration) and the version number and ID change every time the workflow is updated.

Removing these fields and asking for the workflow name instead of the ID, therefore, would be very helpful, making migrations even easier when using this endpoint.