Right, you can’t swap out one Prefab for another as an override. We get how it would be convenient, but it’s not possible at this time. Potential workarounds are things like deactivating one button and adding the green button instead.
Instead of swaping the entire button hierarchy, what about composing the style ?
Have the style in a ScriptableObject, with a MonoBehaviour on the button exposing a parameter of the type of the SO and applying the style values from the SO to the button.
Finally as an override in the Panel Green prefab variant you simply swap the SO reference to the green one.