The CrazyClient auto nexus uses client side HP, so it will beat the FameBot auto nexus implementation every time. The FameBot auto nexus was never intended to be a good auto nexus, its just there for people using Proxy Only clients who still want some kind of safety.
/con and /connect I think, but you shouldn't change servers this way anyways.
The proper way to change servers is to change it in the settings, save and restart K-Relay.
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Regarding the CC vs FameBot Auto-Nexus. I would personally recommend keeping the FameBot one at some arbitrary low percentage (e.g., 15%) just in case you accidentally use K-Relay with a client that doesn't have Auto-Nexus / has Auto-Nexus turned off.