Log into the hub that the device is currently on.
Go to "Manage” and then “Mobile Devices”
In the “Select Device” drop down select the device you are looking to change.
There should then be a drop down for “Hub” this should currently be set to the current hub. From this you should be able to select the target hub.
You should then be able to click the “Update Device” button which should save these changes.
You should then wait 5 minutes and this stage, after which you should then be able to use the device in assignments.