Converting to Java would be quite expensive. The model that Visual Basic uses is called "event driven programming," whereas Java is object oriented. Technically, Visual Basic 6 does have objects, but the style is different, and I think that converting it would be hell and a massive undertaking. Java is very verbose, and it takes a lot of construction work to do very simple things. As a programmer, Visual Basic is a lot more entertaining to program in because you have near instant gratification. Less than a day of work, and you have something to show for it.
When to use Java:
1) cross-platform support.
2) extensive library support, lots of references to API programming
3) The size of your team matches the effort
4) You have professional expertise with software design patterns. Project must be well-defined.
Beyond that, Java is an aging platform that is somewhat miserable to work with. I have worked extensively with it, and I have to say I prefer the .net platform to Java any day.