Can you switch an old verizon contract phone to make it prepaid?

You used to be able to buy a prepaid phone from Walmart and activate it on your Verizon contract. I bought dad a new LG from Walmart not too long ago and when I dialed *228 and option 3 to activate, it said that they werent able to activate the phone because it has to be active on a prepaid account for at least 6 months. I've always bought prepaid phones but now the Verizon system has caught up. I used to work for Verizon Wireless and I didnt have any problems until now.