You really haven't explained why you would want to switch to Java.
If you really want to work with BPM products like Pega and Tibco and you can only do that by switching your skillset to Java, then I would advise doing what will make you happy. Nothing is worse than being in a job that you're unhappy or unsatisfied with -- it can be downright painful. If you're happy in what you're doing, then you'll do a better job at it, which will make you more marketable and will reward you with a higher compensation (even if .NET developers are making more than Java as you say).
If you're only out of college a few years, then you're young enough to make a few changes in your career path and take some risks. Do what makes you happy - it will pay off in the long run. Don't just consider the money -- money will come if you do what makes you happy and you're good at what you do.
Just my 2 cents. Hope this helps.