Please bear in mind that I am not an investment expert, a lawyer or an expert on fiduciary rules, but...
When making decisions as trustees, the owners of the company are acting as trustees of the pension fund and NOT as owners of the company. As unrealistic as that statement may be, it is a fact that if, while taking steps that fall within the authority of the trustees, the trustees place their personal interests or the interests of the sponsoring organization ahead of those of the plan and its participants, they are committing a breach of their fiduciary duties.
Deciding to move the assets from one investment firm to another falls squarely within the duties of the trustees acting as trustees. As such, it is not driven by the sponsor and would not be a settlor function.