Yes, you can change your driving test from manual to automatic. To change your driving test from manual to automatic, all you need to do is to take your driving test in an automatic car.
There’s no need to inform the DVSA or your test centre of anything, just turn up for your driving test in a automatic car. On your driving test report sheet, the examiner will write ‘B’ (for cars) in the category type box and tick the ‘Auto’ box indicating that the test is being conducted in a automatic car.