On the entitlement portal, there is a "Start Date" and "End Date" of the entitlement.
ISG Proxy and SG-VA would rely on this as the base license.
Sometimes there is a renewal on the entitlement or license which will start at certain date and usually it will be the next day after the old license expired.
On the entitlement portal, it already shows the new license.
There is a requirement to force the update of the new license.
The ISG Proxy or the SG-VA does not really need to wait until the last day of the current license or old license to expire, the manual update can be done prior to the new "Start Date" of the new license and the proxy will be updated with the new license even though before the new "Start Date".