I think you can only change from one accounting system to another when you change your vehicle.
It's to stop you counting 40p/mile on a new vehicle and then when it starts getting old and needing repairs, swapping over to costs. You would have already been given an allowance for depreciation in the first accountng period.
So if your van is in good nick, doesnt use much fuel and you have a good no claims bonus on your insurance, then 40/mile saves lots of paperwork.
But if your insurance is high, the vans going to need a bit doing to it to get it through its MOT and it drinks fuel, then you would be better saving receipts.