Not migrated, but we do a lot of CRUD based applications for clients and have a small dev team so it feels like we're kinda porting it over.
The largest change is dependency injection and Entity Framework. DI is insanely better and just more intuitive. EF is really a move from EF to EF Core which has quite a few differences that can introduce some road bumps.
In my opinion, I've gained the most from moving from HTML helpers in Razor to Tag Helpers. It's basically just components for Razor, but it allows us to make a really nice UI library and cuts down on so much of our markup without having to write a bunch of overloads for the same helper.
As far as MVC vs. Razor Pages, I'm still not in love with Pages. It feels a bit too much like ASP.NET Web Forms did. I still use MVC practices and it's almost the same exact experience. AJAX forms are a bit better this time around, but I'm really waiting until Blazor becomes more viable before I make a jump like that.
Doing a quick skim, it feels like that migration guide should still cover the basics. (I don't think there is a big developer-facing change between ASP.NET Core 2.1 and 3.0; small subtle things like the Generic Host Builder. But you should be fine using the guide here to migrate to 2.1 and you can find simple posts on the 2.1 to 3.0 changes.)
I'm not convinced ASPNETCore 3.0 is ready to be used in anger yet. Perhaps this thread from a couple weeks ago is not representative, but I've seen other similar feedback
The largest change is dependency injection and Entity Framework. DI is insanely better and just more intuitive. EF is really a move from EF to EF Core which has quite a few differences that can introduce some road bumps.
In my opinion, I've gained the most from moving from HTML helpers in Razor to Tag Helpers. It's basically just components for Razor, but it allows us to make a really nice UI library and cuts down on so much of our markup without having to write a bunch of overloads for the same helper.
As far as MVC vs. Razor Pages, I'm still not in love with Pages. It feels a bit too much like ASP.NET Web Forms did. I still use MVC practices and it's almost the same exact experience. AJAX forms are a bit better this time around, but I'm really waiting until Blazor becomes more viable before I make a jump like that.