@MarkD good day!
I'm sorry, but according to the description, existing pipelines using the Hive metastore cannot be upgraded to use Unity Catalog. To migrate an existing pipeline that writes to Hive metastore, you must create a new pipeline and re-ingest data from the data source(s). If the original data sources are no longer available, there is no documented method to migrate or copy the data from the existing pipeline to a new one.
The documentation suggests that the data must be re-ingested from the original data sources when creating a new pipeline, and there is no mention of a method to use data from an existing pipeline as the starting point for a new pipeline.
Doc: https://docs.databricks.com/en/delta-live-tables/unity-catalog.html#limitations
Kind regards,
Yesh