You can move to any option that you wish, Bill or Pay as you Go.
I moved from O2 bill to Meteor pre pay last Christmas, to one of their magic offers of free any network texts and a free 20 euro for every 20 euro I top up.
Dropped my bills from 80 quid a month to 20 quid a month top up. I've actually built up about 50 quid in as yet unused credit!