One approach is to use LEM's automatic recycling mechanism to cycle out those old nodes that haven't connected in a long time. We built this to help people in VDI environments automatically delete dynamically provisioned systems.
Manage > Appliances > Properties
An agent node will only "reconnect" to a previous agent's connection if the certificates match, otherwise it's considered a new node and there's not really a way to work around it. You could offload the certs as a part of de-provisioning and copy them back as a part of re-provisioning with some scripts, but that's about all I can come up with.