How hard is it to convert the new(ish) RC/PF trains to 9V?
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)