There is a great tutorial video on the official Ionic YouTube channel which will show you a best-case scenario for upgrading to Ionic 5 and Angular 9.
However for my simple prototype app that I had made last year (just a demo that let me reorder a list) I tried to update but almost immediately came off the rails.
It turned out to be using "@angular/core": "^7.2.2" which broke the app from ionic serve because it was expecting Angular 8.
Then I tried to run the ng upgrade but got hit with incompatible peer dependencies for codelyzer, @ionic/angular-toolkit, and @angular/http.
So I tried to upgrade to Angular 8 first with npm i @angular/core@"~8.x.x" but that failed as it needed npm i zone.js@~0.9.1.
How can I update successfully to the latest and greatest?