How hard is it to convert the new(ish) RC/PF trains to 9V? I have a HUGE investment in 9V trains (think many thousands of USD) and was wondering how difficult it is to convert the newer RC/PF train systems over to 9V motors? Do the 9V motors fit? Does the base have space for the trucks? Is it even worth it (keep in mind, I hate batteries)? Converting the 12V trains to 9V was rather easy.

I haven't ventured out to purchase a set yet (even though they've been out for a few years now -- I'm lazy and cheap I guess) because they're all rather expensive (as we know), so I have not been able to check myself.
The PF train motor has the same size and shape as the previous 9V Motors, and a lot of people are happily substituting one for the other.

The Emerald Night, however, doesn't use that motor, so there your job will be much more tricky. Usually people change the tender to add two 9V motors there, but the wheel train of the engine tends to derail fairly easily if pushed that way (I strongly suggest you read RailBricks issue #7 for tips on how to make it run smoothly)