You're correct in that if your licence expires between 1 February and 31 December 2020, it was extended by 11 months. If it was only changed 4 years ago and you don't look too different, I wouldn't bother changing the photo - which means you should be able to apply using the online DVLA service. It's certainly quicker than a postal application for an over-70s licence. You can see what you need to do here: www.gov.uk/renew-driving-licence-at-70