Get the 2 part reef epoxy, like you probably used to glue your rocks together and mix it and they instead of turning it into balls like when you want to mount frags or join rocks, you roll it out as flat and this as possible and then just place it over the top of the corals you want to cull and push it down. It might take a couple of times to find all the nooks and crannies the corals have grown into.
After a month if you want you can pop the epoxy off and then you will have a dead coral skeleton base - which might look more natural - for mounting your new non-sps corals to.
Dave B