> But what if you have an established app with real users?

I recently used GPT 5.5 "extra high" running nearly non-stop for about a week to upgrade a legacy ASP.NET Web Forms app to ASP.NET Core on .NET 10.

This was considered "too hard" (too expensive) for human developers because it is a wholesale rewrite of every web page template. Not to mention that the dependency injection mechanism is totally different, async is more pervasive, etc.

Worse still, the old app was split into a bunch of components with a variety of web API protocols in between them, had stupidly complex Oracle stored procedures, and a whole series of hidden land mines in the codebase. It was an undocumented, unmaintainable mess full of dead and spaghetti code.

I would have estimated 6 months minimum for a human developer to uplift it, but 12 months is more realistic.

Doing the same in a week feels like tapping into some sort of forbidden black magic. It feels like I can't admit this to anyone, lest they think I dabble in the dark arts.