"[M]any 403(b) plan sponsors have tried to freeze flows to their legacy annuity providers and designate just one or two providers moving forward. If the plan sponsor hopes to map legacy 403(b) assets into a new lineup of mutual funds, the process can be especially challenging, as that move has to be directed by the participant. Participants invested in these assets often have full discretion over their money, meaning that plan sponsors cannot force them to move their funds."