If you’ve had a 12-month SIM only deal for at least 3 months, you qualify for a SIM only flexible upgrade.
If there are any outstanding charges on your SIM only bundle when you reach the end of the third month, you’ll need to settle these payments before you can upgrade to a Pay monthly mobile.
You check whether you can upgrade to a Pay monthly mobile in the My Vodafone app. Just go to Upgrade or change plan in the main menu. You'll then be able to upgrade from there, or be told when you're eligible for an upgrade.