It's really easy to change but has to be done at the Post Office or DVLA, wait until the vehicle is sold and ready to tax, take the V5C to the POst Office completed with all the new keepers details , circle the 'disabled' part with a black pen and then draw an arrow from there to section V.5 on the bottom of the first page just above the green section where it says 'tax class' write in petrol car or diesel car. PO will tax it as you have asked petrol car or diesel car, then ask them to post the doc to DVLA, it won't even cost you 50p for a stamp