In most cases, it is not necessary to remove all the old grout before regrouting. However, it is important to clean the surface of the tile well so that the new grout will adhere properly. The easiest way to clean the tile is to use a muriatic acid or hydrochloric acid cleaner. Be sure to wear goggles and rubber gloves when using these chemicals, and follow all safety instructions on the label. If you are not comfortable using these chemicals, you can also use a commercial grout cleaner or a bleach solution (1 cup of bleach per gallon of water). After cleaning the tile surface, let it dry completely before beginning the regrouting process.

No, you don't have to remove all old grout before regrouting. Depending on the condition of the old grout and how badly it's stained, you may be able to get away with just cleaning it well. If the old grout is loose or crumbling, then you'll definitely need to remove it before installing new grout.