CAVEAT - this is only for those who are due a D4 medical renewal and have not had any licence issued by DVLA after October 2012
For those who are due a D4 medical renewal there is a weird EU thing where the issue of the new style driving licence automatically extends the LGV & PCV expiry for 5 years FROM THE DATE THE NEW LICENCE IS ISSUED and not 5 years from the current expiry date
This can be done by simply changing the current photo on the licence even if it has not yet reached its expiry date
From November 2017 this will not be possible as everyone should by then have a new style driving licence
I did mine by using the easy Post Office driving licence photo renewal service where they take the photo for only an extra nominal amount =
You’ll need £21.50 in total. The DVLA Driving Licence Expiry fee is £17 and our renewal service fee is £4.50
To find out if you have a PO handy to you for this service try this - postoffice.co.uk/branch-finder - put in your location and tick the DVLA Photocard Renewal box
The POs that offer this service will have the D798 photo renewal form
The photo can also be changed using the online DVLA service where they take the new photo from a current valid passport gov.uk/renew-photo-driving-licence
Medical worries in not doing a D4
Many have voiced genuine concerns over this but its easy to remedy
Take the free NHS health check
Ask GP if they think there are any driving health issues
Get free eye check - plenty of offers these days from various opticians
Many on this site including me have already benefitted from doing this 
I know I have posted this before but it was some time ago