Generally you can move the robots by just selecting the robot in IM ->right click-> mode to required hub.
Secondly you can try creating a robot_update package with new hub name and IP and deploy it on robots which should report to this new hub.
But i believe you may face network and communication issues if you try to report servers from on premise environment to hub hosted on Azure. I suggest to let all such robots report to a hub in your on premise environment and then pass on the alarm/qos data via tunnel&queues to the primary hub.
you can go to below tech article for more information on having a hub hosted in azure
Tech Tip: UIM - How can I deploy a remote/secondary hub in Azure?
Note: Please test this on few robots first before mass deployment to avoid any risk.
Any other suggestions or simpler solution guys?