Well, keep in mind that .net core and .net framework?
From a coding point of view, you REALLY can't tell the difference.
So, if you
c#, vb.net console framework -> c#, vb.net console core?
No real changes.
c#, vb.net windows desktop framework - > c#, vb.net windows desktop core?
Again, no real changes - you not notice the differences
However, with web based applications?
Then if you have say c# MVC framework?
Again, to .net core? Really VERY few changes - and again you not notice the difference.
However, a webforms .net framework?
That's not going to convert automatic, and you need a near 100% re-write. And the reason is that .net core has no support for web forms.
And 2nd to above, there is no vb.net choices for MVC core. (you have to use c#).
So, for an existing MVC application? Then the referenced migration assistant pointed out above should do fine, and minimal changes will be required (in most cases).
However, unless the web site application is a MVC application, then NONE of those tools are really going to apply, nor help you much in regards to a migration project.
Since webforms is so VAST different then MVC, then that web form site no matter how you spin this will have to be re-written from scratch, and converted to a MVC project, (or a have been a razor site).
So, if this was/is a web forms site? Then you need near 100% re-write.
And if this is/was a MVC site written in vb.net? Then that site probably should be converted to c#, and THEN you migrate to .net core.
So, the .net core runtimes work like, look like, seem like and for the most part work VERY much like the .net framework runtimes. The only major change from .net framework to .net core is that .net core has the windows dependencies removed, and thus of course those runtimes thus are quite much CPU and even OS neutral to a far greater extent then the .net framework (which over the years did become relient on windows features).
correction: The migration tools DO WORK for webforms.
So, what occures is a .net core web site is created that is placed in front of your asp.net webforms application.
The result is this setup:
this is VERY ideal for a migration project, since then you running a .net core site, which runs in "front" of your asp.net webforms site. That THEN allows you to start (over time) moving pages from webforms to MVC (or blazer syntax). However, there are MANY questions we see of people attempting to run the webforms site "side by side" with the newer .net core web site. The above setup solves this issue in a much better approach, since in place of "side by side", you wind up wit a setup in which the .net core site runs IN FRONT of the web forms site. At that point then, develoeprs are free over time to start moving those webform pages over to the .net core app, and do so over time (which of course is ideal).
So, while the conversion tools can't "convert" web forms, I stand corrected in that the migration assistant tools does not support nor have anything to do with webforms. The migration tools MOST certainly does support webforms. Of course the above diagram shows that no real conversion of the webforms occures, but it does setup for you a .net core site that runs in front of the existing webforms site. Then, over time, you can re-write and start moving single pages 1 at a time over to the newer MVC design approach.
the above diagram, and link on how the migration assistant works is outline here: