Have you already activated your phones? It's possible that the plan you want isn't compatible with your phones. Not all phones work with all plans.
Let us know the make and models of your phones, your current plan and the new plan you want. Or, call the experts at Customer Care at *611 or 800-922-0204. If you're increasing your minutes and/or adding unlimited messaging and/or data, it's important that the plan is backdated to prevent overages.