need to swap my iphone 4 on vodafone for one on o2 asap!!! ?

need to swap my iphone 4 on vodafone for one on o2 asap!!! its black 16gb in good condition, works on vodafone but im on o2, i will even swap for a iphone 4 8gb in white or black, must be in good condition, we can keep our own accessories and jus swap over the handset, im in cheshunt/enfield 07701003320