( Flutter and Mac ) Possible to create a new project to upgrade an existing app?

I would suggest saving the project in version control, delete the current macos and build folder and regenerate it with flutter create . Then you can add back the code and config you had before in xcode. This way you will keep your current flutter project but with a new xcode macos project.

Before that, you might wanna try if calling flutter clean solves this problem. It also cleans the Xcode project.

For keeping the app you just need to have the same bundle id and a valid signing certificate. https://developer.apple.com/forums/thread/39216