Why do you want to move them from a frag plug to a larger frag plug or ceramic disk?
Once they are encrusted on the disk, and rolling down the disk, I just glue the disk to my rockwork and let it encrust onto the rocks. Eventually it will cover the disk and you will never be able to tell there is a frag plug there.
Once they start to encrust, it's best to just leave it be or risk killing it by removing it from the plug. Some corals can recover from being ripped off the encrusting plug, while others will just die. If you want to move them to larger disks, it's best to do before they start to encrust. Some corals you can slide a razor blade under them and get everything off the plug while not hurting the coral one bit.