I recently bought a Surly cross check for commuting, light touring, and some road riding. I'd like to get some lower gear ratios going for the steep hills around where I live and for when I do load the bike. However, it looks like converting the bike to a triple will be sort of $$$ at the moment so I was going to try to simply change the cassette and see where that gets me. The rear deraileur that came with the bike is tiagra. Will that work with a 9 speed deore cassette or will I need to replace the rear derailleur as well? If anyone has any thoughts or other ideas, I would really appreciate it.