You might be able to change after 15 months, not sure about after 12.
They won't let me change to the dolphin plan as I signed up to an 18 month contract the week before dolphin was announced.
Proper dirty trick by them IMO, offered a lot of people early upgrades to stick them on 18 month contracts so that they couldn't change to dolphin when it was announced.
You get 14days to cancel.
I signed up to a higher one and phoned and said I wanted to change to Dolphin. She said it wasnt possible. I told her to save the hassle of me cancelling, sending the phone back and then starting a new contact and them sending me the same phone again, they might aswel let me. and they did.