Unable to convert a standard Unity prefab into a uPrefab.

[Carlos Mangas]

The asset needs some way to convert the existing prefabs into uPrefabs without breaking the references between them, right now it's unusable for people who already have a great amount of prefabs.

[Devon Klompmaker]

You can simply right-click each prefab in the project and "re-import asset". This will add the needed uPrefab and uPrefab Child components. For nested prefabs within existing prefabs, you can add the uPrefab component manually and assign the reference by hand.

Does this resolve your issue?