Is it expected behavior to lose OneAgent connectivity on previously monitored hosts when OneAgents get installed on new hosts which require more host units than allowed in the that go over their license limits?
The license does not allow overages.
In general, if you install new host it will connect normally for around 30 minutes (as I remember). After this time if license will not be available it will move to unactive state because of license quota reached. If you will restart hosts that are monitored it is possible that that out of license one will replace them in monitoring. There should not be such option that new host will eat license from operating ones without OS restart.