Just a note to myself. I picked up how to do this from http://vmwire.com/tag/esxi-vcd-agent-uninstall/ (thank you Hugo).

I was in the situation where a vCD install had been trashed by an I/P re-address. When re-installing vCD, it couldn’t put its agent back on the hosts because the old one was still there.

I’ve now removed the old agent as described above, rebooted the host and “prepared” it from vCD.