Yeah all that is completely obvious. So why not just communicate honestly: This is a massive breaking change. It is going to be painful, but is necessary to moving forward. But don't deny the reality of it.
Saying .net 5 is unifying .net is like saying Python 3 unified Python or that Angular 2 unified Angular.
> If you haven't been keeping the app up to date, say you're on 4.6.1,
Again, why the dishonest arguing? 4.6.1 is years old, but you will have exactly the same problem if you are on 4.8 which was just released. It has nothing to do with code not being updated.
> where you have an old neglected project that you want to bring up to date
Again, this has nothing to do with projects being old and neglected. It is a breaking change, it will affect all projects on .net framework if they want to upgrade. And you know that.