There are specialty tools designed for removing old, stained, cracked tile grout, but few work as fast or as well as the multitool. Attach a carbide-grit grout-removal blade, and use it to grind grout from the joints between floor tiles and wall tiles. Since the blade oscillates back and forth and doesn't spin like a grinder, it doesn't spew grout dust throughout the room. Grout-removal blades come in different thicknesses for use on narrow and wide grout joints. - PopularMechanics.com
Great site for DIY... Removing and replacing old grout. First Remove the old grout…. If the grout is already coming out and is fairly loose – Use a grout removal tool found at all home improvement stores. Scrape along the damaged grout line and dig the old grout out of the grout joints.