Marc,
You cannot change over to 40p per mile unless you change your vehicle, whatever method you choose must remain with that vehicle until you change to a new one.
It's 40p per mile for the first 10,000 miles then 25p thereafter.
The reason why I went this route is in my first year I ran both methods in unison and found out that I was better off the 40p per mile route. Accountants will not adivise that route because they get to gain more money from you by going through your receipts.
I do around 14,000 miles a year min so from that I get to claim £5,000 as my vehicle expense. I cannot claim any other thing against vehicle expense. My actual running cost fuel, tax . depreciation etc.. come to around £3,000 a year so the tax man gives me £2000.

But each person is different so it may not work to your benefit, I did do myself a spreadsheet and was surpirsed that if I bought a new vehicle in time it pays to go the 40p per mile route which I was surprised.
Hope this helps.
Doug