They invested time and money to write 3 million lines of code and they're will to scrap it? This sounds crazy, there must be something fundamentally wrong with the application to begin with, I think they should re-visit the design model and figure it out.
The only viable solution that may come close to .NET is J2EE, read this article and judge for yourself - keep in mind that it comes from a Java shop.... http://www.exia.net/html/app/sitecontent/CGI%20Dot%20Net%20vs%20J2EE.pdf