Hmmm, so it's a new bike equipped with Ultegra 10-speed, but the decision is being based on whether you can use a 9 speed cassette you have? I'd say start checking to see if there's a 10 speed cassette that's similar to what you already have (there likely is). And as JiveTurkey mentioned, you'll most likely need to replace the RD with a mountain bike version. You can probably get the combo for a hundred bucks or so.