You can absolutely slowly migrate a UIKit project to begin using SwiftUI – it's part of what I've been doing at work for the past 8 months.
The way to cross the boundary from UIKit to SwiftUI or SwiftUI to UIKit (either direction) involves implementing one or maybe two classes/structs and is not too difficult to do.
The way to cross the boundary from UIKit to SwiftUI or SwiftUI to UIKit (either direction) involves implementing one or maybe two classes/structs and is not too difficult to do.