If you simply change the cassette, you will DEFINITELY need to change the chain as well... otherwise you will find it skipping especially with as much pressure as you will need to climb! If you do change and put a compact crank on, as long as you use Shimano, you should be ok and not have to change the front derailleur... otherwise you may have to change that as well. As for losing gears with a compact crank, it's definitely worth the change... your top end gear doesn't get used very often anyway. Nashbar had a compact crank on sale for a very reasonable price... I had a custmer that purchased one... remember though that you willneed a new bottom bracket as well.

Good luck with your ride, sounds lie a great one! We have one her that climbs Mt Washington... it's ranked (professionally)as one of the ten tughest races in the world...