they are quite different. but not so much that you couldn't re-train if you already know VB6.
the .net language basically seems to have just addressed all the bitching and moaning that people used to do about saying that VB wasn't a real language, classes are much more apparant as classes and the syntax of the way things are done, is (in places) significantly different!
personally I don't actually thikn it's an improvement, but some people do...
if you do a search on the differences apparantly some news reports suggest that a lot of people just gave up with VB, (which was the most widely used windows application programming language), and switched to C# as a windows programming language...
certainly, for me, (a home developer), the switch basically meant that VB was now new, unfamilliar, and not worth re-learning.