We're in the process of migration a large MVC application from Framework 4.8 to .NET 6. It's been....painful. Hopefully at least for a while updating to new LTS versions will be less so.
We did that last year on a 3 million line / 500 person-year investment application. Quite challenging as everything from app domains to remoting, to call contexts and other unsupported things were being used. It was definitely worth it for performance alone as we were also using Mono to run on Linux.
I am moving a personal project with ~50k loc and it has turned into a rewrite of about 2/3 of it. Not fun, but on the plus side the app runs A LOT faster now. So much faster that even non tech people that use it noticed the difference.
We are also preparing for a move from .NET Framework 4.7.2 to .NET 6/7. It's going to be quite the project.