Greetings,
Presently, it is not possible to change the default pinning policy. Its fixated to OnLock and will float until the item in question undergoes a lifecycle and revision change process via change order workflow and the workflow enters a locked WF state. At that point, whatever latest file version was float to will be pinned and the item will not consume future file versions. Once item is released, that very same version will become persistently pinned to the newly stamped item revision. I'm curious what business use case are you trying to solve by changing the default pin policy. It doesn't have to be the case with your situation, yet I'm assuming the item would undergo a revision process, floating a file version while item is undergoing a CCB process and people are reviewing is moving the goal post and folks may cast approval on different content. This is the main reason onLock is hard-default.
Does this help?
Martin Gasevski | Fusion 360 Team Product Manager