Yes, it’s true, for many years the IO-550-F upgrade was indeed done with a 337 field approval. The -F model is actually very similar to the IO-520-A so there’s no changes to the airframe. Just new baffling needed.
Today, Lone Mountain Aviation as an STC for this upgrade: https://lonemountainaviation.com/ . Because of this, 337 field approvals are no longer granted. Fortunately, the STC itself isn’t very costly.
The biggest cost in the upgrade is mainly the core up-charge.