You say it was a year? You may be required to pay what you didn't pay them the first time, plus any other fees, including late fees and collection fee's.
If you want to get a prepaid account, you would have no problem using the phone you had before, just buy a started kit. To go with a new Flex plan, not sure how well that will work out. I don't know how it works, but i'm pretty sure that if you didn't pay a year ago, it won't be easy to come back in.
Maybe someone that has Flexpay can correct me if I am wrong, but I think this is how it works bud.. Sorry