Flooring Tile - Best way to remove old grout

I have a light colored grout in the kitchen that I would like to remove and regrout with a darker color. I've tried stains/sealer but it still traps the dirt. What is the best way to remove old grout. It's about 400sq feet of 12X12 tile with 1/4" grout. Speedy removal for me doesn't really matter, I have all the time I need to do this project. Once I remove a small section of grout then can I just use a hammer and some type of chisel?

You are talking about a HUGE undertaking. WIth that much square footage and 1/4" joints it would be easier to remove the whole floor or if possible go over top of it with new tile. Also you would likely damage the tile trying to chisel it out.