Apple give you the option of switching from pay to download and free as much as you want, this give you the opportunity to have sales or make the app free for a certain amount of time, or just change the free app to pay to download when you have a user base.
How do I make the app pay to download?
1. Log into your iTunes connect account.
2. Go to the app you want to change price on.
3. Press Pricing and Availability
4. Change price and time availability
Set up prices & app distribution
With your Play Console, you can set your app as paid or free, select your app's country availability, and opt-in to distribute your app to additional Android devices and programs.
Make your app free or paid
1. Go to your Play Console.
2. Select an app.
3. Select Store presence > Pricing & distubution.
4. Next to "This application is," select Free or Paid
Changes to your app's Free or Paid selection
- You can change your app from Paid to Free.
- Once your app has been offered for Free, the app can't be changed to Paid. If you want to charge for the app, you need to create a new app with a new package name and set a price.
Note: It is not possible to get your already published free app changed to paid. You need to create a new app so it doesn't have the same package id and publish that one with a price from the beginning.