The grout in our shower seems to have a few gaps, so I think it needs to be replaced. It is about 18 years old. The grout is quite narrow, 1/8 " or so. I am pretty handy, but I don't want to damage a lot of the tiles while trying to save a few bucks with my labor.

Is replacing the grout a homeowner job or best left to the professionals? If I do it myself, is it better to remove all the old grout or only repair areas that seem damaged?
Thanks for any input!