All the threaded inserts I've seen were right-hand thread and either a straight slot or hex socket for driving. Might be expoxied in which case some heat to soften perhaps. You could drill them out; might even pilot a small diameter hole saw in the insert and cut on the periphery and pop em out whole--bigger hole to fill tho. It's brass, how hard can it be? Glue in dowels and establish new centers.

jack