removal of media in filter would really be risky IMO
as that resin decor really isn't very pouris . If your adding a new filter add the old media along with new media till the new media is seeded 2/3 weeks.
but if your going to add a few pieces of live rock I would try to get at-least 10 lbs
wait a few weeks than add the new filter that would be the safer way to go
9 fish in a 20g that's way to many , don't take this the wrong way that coral beauty has no business being in that tank as they require a much larger tank . I would think him alone would over load the bio load
I would suggest a bigger tank or re home him .