The DVLA will contact you a few months before your licence runs out. They will send you a form to complete and ask for a medical opinion as to your fitness to drive, and of course questions will be asked about your eyesight.
Don't worry, as as soon as you send your form to the DVLA, you are still covered to drive while they are dealing with your application. Dont worry though as I've been a Type 1 diabetic for 60 years and have been relicensed every 3 years. I hope you'll be okay. It is something I live in fear of every 3 years. I feel we are being prejudiced against, but do know that people with other medical conditions need to honestly fill out these questionnaires. Good luck
Natalie