Provided your credit rating is reasonable:
Place a goat's entrails on the keyboard at sunset when the moon is new. Face North West wearing a t-shirt back-to-front and inside out, mutter an incantation to the fairy-people and pray.
Seriously, you will need to get what is called a migration code from your current PAYG company and then choose a new service provider. The process can be more trouble than it's worth (hence my earlier levity) and I opted for a 'ghost' number that I own outright and give out to anyone who needs it. This number is used to diivert to my mobile of the moment.