Pay & Go to Prepay Flex is extremely easy, but then when you go to reverse the process, there is no website option. presumably you have to mess around calling their customer support, although their Pay & Go website says "If you’re an existing Prepay customer, all you have to do is change your plans and you’re good to go" followed by a "Change Plan" button that just redirects to My Vodafone. If there's an option there, I can't find it (it was easy to find when switching to Prepay Flex).