Implementing Free Trials for My Paid App

I have few questions about implementing free trials for my UIKit Swift App.

Currently my app is paid i.e. One time purchase.

I have good amount of user who have purchased my app as an Paid app previously.

Now I want to introduce Trial period for my app

I am also exploring RevenueCat framework for this...

My Questions:
  • If I implement Trial period then can I offer free app as my previous user have already bought my app when it was paid.

  • And new user can use my app for free for 14 days then one time payment will start.

  • Is there any good documentation for this kind of logic / programming? with source code examples...

Thanks in advance...