Answer:
Just submit your renewal application in plenty of time (which is usually 16 days before permit expiry), for the NHVR to get your application to the road managers. Note that you are able to apply up to six weeks before your permit expires.
Once submitted, your application needs to be with the road managers for a minimum of 14 days, and if no response is received by NHVR the permit can be issued.
If the application is submitted after the expiry date of the permit, the NHVR is unable to expedite the application and it will be treated like a new application.