How do I implement Receipt validation to transition users from paid to free+inapp?

HI, I have a paid game and want to transition to free+ 1 inapp to unlock all the content. I dont want my current users to have to pay again and I read about receipt validation. From what I understood y you read from apple's servers if the player bought the game and then unlock the content. I have iOS native but it doesnt support this. Any ideas how to implement it without going into objective c? Thanks.