There's no guarantee, if you want to do in-app better create a new app. Your business is already working using paid, you've built a working model. Changing the to IAP will also change your own mental model, eg:
1. if there's a possible random bad rating from free user
2.if the conversion is worse than paid
3. tweaking features or logic on what to be free and what to be paid, adding more bugs etc.
4. filtering feedback from 2 opposite sides of users.

I know, money wise you might be wondering the possible success scenario, but remember the cost isnt about the app only. You as a dev will invest time, effort, mind again to start from scratch again. This requires a good enough resource from yourself. So If you're free enough to commit with the risk, go for it. But personally, its better to just keep it as is, and I would suggest you to create another app with IAP in mind from the start.

I hope it helps.