You might consider calling the Arkansas Income Tax number directly, explain the situation, and ask them if you should file an amended return. When I called them to ask for guidance they informed me that they would make the filing changes for my return based on the tax law change. That way I would not have to file an amended return.
Fortunately, I delayed my Arkansas filing until last week after the TurboTax update and saw the benefit of the tax law change. It was a significant surprise to me. I received a return as you mentioned versus having to mail the state a check.
Good luck. Maybe someone smarter than me has some better advice.
