At this time, not all customers can transition to monthly billing. If you're on the new infrastructure, your renewal date is less than 30 days away, and auto-renew is turned off, check the Domain Settings > Billing Information tab in your Google Apps Admin console for a Learn more link to see if you're eligible.
We currently don't offer a way to manually switch billing systems in the middle of your subscription; however, we will begin automatic transitions to the new system in late 2011. The automatic transition to post-pay billing does not have any service or billing impact on customers who have a pre-paid annual contract. If you have selected to auto-renew your annual contract, it will be auto-renewed to the Annual Plan, with monthly payments, once accounts are migrated to the new billing system.