Here is an adaptation of my answer to a similar question.
Converting large programs automatically is a better choice than rewriting. It's a common pitfall to start out optimistically rewriting a large piece of software, make good early progress fixing some of the well-known flaws in the old architecture, and then get bogged down in the functionality that you've just been taking for granted for years. At this point your management begin to get twitchy and everything can get very uncomfortable.
...and here's a blog post by a Microsofty that agrees with me:
Many companies I worked with in the early days of .NET looked first at rewriting driven in part by a strong desire to improve the underlying architecture and code structures at the same time as they moved to .NET. Unfortunately many of those projects ran into difficulty and several were never completed. The problem they were trying to solve was too large
This excellent Microsoft page recommends two third party migration tools as better than the underpowered built-in VB.NET upgrade wizard - Artinsoft and CodeArchitects VBMigration. Artinsoft wrote the built in VB.NET upgrade wizard, this is their improved version. And CodeArchitects was founded by Francesco Balena, who wrote some of the classic books on VB6 and VB.NET.
The same Microsoft page also says:
Performing a complete rewrite to .NET is far more costly and difficult to do well [than converting] ... we would only recommend this approach for a small number of situations.
EDIT: Sung says in the comments: "I am not a big fan of auto generation of code because it is harder to debug initially and might take just as long as it takes to rewrite whole thing". I have to disagree strongly. In general I too am no fan of code generation, but in this case the resulting code will be structured identically to your original VB6 and should be almost totally functional. I haven't actually tried these tools myself yet, but from their customer testimonials this promise is fulfilled.
And I repeat the Microsoft advice just above, based on their experience of assisting many migrations - "a complete rewrite is far more costly and difficult than converting [my emphasis]" - a flat contradiction of the supposition that it might take the same time. If you want to improve the structure of the VB6, migration then gradual refactoring is likely to be far more cost effective than a rewrite.
