Correct, once you convert a BSP to a static mesh you cannot revert back. Making any changes would involve exporting the static mesh to 3D modelling software (like Blender or Maya) or starting from scratch.
As for your prior question. Technically you don’t need to convert the ball into a Blueprint first, the student-provided solution does that but it’s not the only way. See below code, added to Begin Play in the level blueprint.
While this code works, it gets clunky if you have multiple levels or wanted to setup multiple balls.
As for when to use a blueprint or not… The big advantage to using blueprints is that they’re reusable in multiple levels without having to rewrite code, or if you want to have multiple of the same object with the same functionality in your level.