Steps to reproduce:

1. Create a monobehaviour referencing a Material
2. Assing the material to Default Diffuse or any built in material, create a prefab
3. Open the Render Pipeline Converter.
4. Click in Materials Converter, initialize. 
5. See that the prefab is shown on the UI.
6. Click convert. See the prefab again, still references built-in material instead of URP one.

Actual results: Prefab references built-in material

Expected results:  Prefab references URP material.