No. Regrouting is not a band-aid solution, so you should never apply a new formulation on top of a damaged grout. The new grout will not adhere to the tile edges, resulting in early cracking and deterioration. It will also bring you back to square one, which is a complete waste of time and money. Since you are not getting rid of the stained and discoloured grout, it will still appear underneath and compromise the look of the entire surface.