You probably don't need to regrout.
First, you should know that phosphoric acid is typically the active ingredient in bathroom cleaners because acids will cut through soap scum like a hot knife through butter, but phosphoric acid won't harm chrome. Also, phosphoric acid, along with hydrochloric acid, is what's typically used in toilet bowl cleaners. Hydrochloric acid is much more aggressive than phosphoric acid at the same concentration, and will damage chrome, and just about everything else, too. Given time, phosphoric acid will dissolve anything you'll normally find in a toilet bowl.
I own a 21 suite apartment block, and what I've found works well for me if and when I need to clean grout is a three step process:
1. Use an phosphoric acid based TOILET BOWL cleaner to dissolve the surface of the grout, which you can get at any Janitorial Supply store listed under "Janitorial Equipment & Supplies" in your yellow pages phone directory. The toilet bowl cleaner will be gelled, and that helps the stuff stick to the ceramic tiled wall. Look for a phosphoric acid content somewhere between 15 and 20 percent. Don't be too aggressive cleaning with acid as you WILL be dissolving grout, and you don't want to dissolve very much. You just want to dissolve the surface which is all clogged up with soap scum and mildew. Use an old tooth brush or a nylon bristle denture brush (or even a grout brush) to scrub the grout as the acid works. Work in small areas from the top of the discoloured area down.
2. Allow the grout to dry overnight. Then scrub the grout with bleach straight out of the jug with an old tooth brush or a denture brush. Time is on your side on this step, as the longer you leave the bleach on, the more deeply it penetrates into the grout. Scrub with pure bleach every 15 minutes or so for the first 2 hours, and then every half hour or so for a few hours, and then every hour, and you may have to carry on the following morning too. Most of the change will come within the first 15 minutes to half hour. If you see no change after a few hours, then I'd give that spot another scrubbing with acid to open up the surface porosity, and try again with the bleach after letting that area dry for a day or two.
3. After you get the grout white, (and replace any grout you couldn't get white, SEAL the grout with a good grout sealer to prevent having to do this job again. I use a film forming sealer on my grout, and one of the advantages is that it allows me to use phosphoric acid toilet bowl cleaner on my ceramic tiling to remove soap scum. Because there's an acrylic film over my grout, the acid cleaner never comes in contact with the grout. The sealer I used to use is no longer available, but one similar to it is available as "Grout Sealer" from a company called Glaze N' Seal in California.
http://www.glaze-n-seal.com/images/lg_grout.jpg
http://www.glaze-n-seal.com/docs/fac...ler%20FS41.pdf
So, while you do remove some grout using my method, it works well and you do seal the grout afterwards so that you should only have to do it once.
If the surface of the grout that was previously clogged up with soap scum and mildew is completely removed, you could probably also use a penetrating sealer instead. But, if the scum and mildew hasn't been completely removed, you might not get very good penetration of a penetrating sealer. The Glaze N' Sealer is very hard and lasts a very long time. The grout sealer I used to use recommended two coats, but all of my bathrooms have closer to about 6 to 9 coats of acrylic plastic over the grout. However, I also have bathrooms that were tiled 20 years ago that have no mildew at all on the grout.
What works and what doesn't to clean grout?
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