With ArcPro 3.5, ESRI has cracked down on letting us define the projection on a feature class that has a projection. We have been importing an existing schema from an XML and then using Define Projection in a model to redefine the projection for the new project. In 3.5, this no longer works. We tried it in arcPy as well.
This seems like an unfortunate level of hand-holding. ESRI should trust users to know when it is appropriate to redefine a projection.
