You would have to send 30-day notice to state that the account they are in is no longer being offered and they will be converted into a new type (be sure to include the new TISA disclosure). If you are sun-setting that product type, you convert customers into a different type... I am not aware of any requirement to convert them from a NOW into a NOW.
However, if there is an interest bearing account that is similar to what they currently have, I would HIGHLY recommend you convert them into that. Otherwise, as osucpa said, you're going to have some hella peeved customers.