Remove old light box with a stud and nut anchor
These hangers were typically used in ceilings where they wanted the light to be somewhere which required space or location between joists. The bracket is typically nailed to both opposing studs or usually under the plaster or drywall.

You could cut it with a hacksaw and move it out of the way. Or for more work, cut a separate hole for the recessed light and then patch the old hole from behind.

You mentioned "retrofit". Retrofit usually means a prior recessed light fixture, which this is not.