It was a horrible mess, everything had to be re-written to be compatible because it was really an entirely new language. Developers were left in the lurch
I worked for a company with large amounts of VB6 code, certainly over a million dollars worth of developer time. The company simply could not afford to do a rewrite. I still can barely believe that Microsoft would screw so many businesses by doing that.
I vividly remember everyone gathering around as I used the project conversion wizard to convert from VB6 to VB.NET. I knew it wouldn't really work due to fundamental differences in the languages, but I felt we had to go through the motions. We sat there and watched the estimated time of completion climb and climb and climb. Eventually I think it just crashed.
At the time we had a big meeting and I tried to convince the company that we should move away from Microsoft technologies to avoid similar pain in the future. However, such is the "Gravitational Pull" of Microsoft products, we ended up sticking with them.
Of course, if there had been a compatible rival product (some kind of Borland VB) then it would have been much harder for Microsoft to abandon the language. This could be an issue if they try to abandon