Hello, I am planning to significantly update an existing app in the Apple Store. The app was originally developed using Flutter, but now I plan to rebuild it using native SwiftUI. Will the review process reject the update due to the significant changes in the app?
You won't get any official answer about the App Store policies here, but the App Store reviewers look at the actual app experience, not at the way you built it.
Each update you send will be reviewed as usual, it doesn't matter if you change the whole thing or not.
Will the review process reject the update due to the significant changes in the app?
Not at all. Apps do evolve significantly in new releases, either functionally or technically. They don't mind as long that you comply with guidelines. You don't even have to mention the change from Flutter. In fact it would probably be seen as a positive move as more future proof and guaranteed multi platforms.