You may encounter this warning when changing a wall's type to a curtain wall type, and the wall has permanent dimensions.
Warning: Switching wall type to family curtain wall. All dimension references to the side faces of the wall will be deleted.
Issue: You have placed permanent linear dimensions between wall faces and then changed one of the walls to a curtain wall. You lose those dimensions.
Solution: You will not lose dimensions between wall centerlines, if you change one wall to a curtain wall.