"Beginning May 27, 2012 if you are using the $25, $40, or $60 Beyond Talk plan and you upgrade to a Beyond Talk Smartphone launched after May 1, 2012*, you will need to switch to the $35, $45, or $55 Beyond Talk plan.
When you switch plans, you will need to pay for your new plan and you will lose any minutes, messages, and money in your balance. This will also change your monthly payment date. So if you are considering upgrading to a new smartphone, we recommend that you wait until right before your next monthly payment date to activate.
*The following smartphones launched before May 1, 2012 and are therefore not subject to the new device upgrade rules at this time: Venture, LG Optimus Slider, HTC Wildfire S, Motorola Triumph, LG Optimus V, Blackberry Curve 8530."
From my official portal.