replacing flaked out areas of grout above tile with polyurethane caulk

Hi,

I noticed that there are several small areas where, along the tops of my tile, the grouting has flaked away. Where there is grouting missing, you can see the top of the tile, and of course this is directly below the underside of the coping. So pieces that have fallen out (all rather small really) are grout that was right up against the tile. It has been suggested to me on another forum and also by my pool builder (the pool was just fully renovated two years ago so I really didn't expect this sort of problem but apparently there has been some shifting that has occurred) that the grout be chipped out and replaced with polyurethane caulking. I see the advantages to this being that the caulking remains flexible, but I'm wondering if it would be better to simply fill in the missing gaps with some caulk rather than go around and chip out all the new grout. To do so could conceivably damage the tile that is directly behind the grout or even create further problems. Basically it is only a few tiles that show the flaked grouting. The rest of the pool looks fine except that a very fine separation line has developed in some areas between the top of the tile and the bottom of the coping. My builder went around with a hammer and said the tiles all sound solid but that it does sound hollow under the coping (another thing I would not have expected with a 2-year old reno). So it appears that whatever shifting has taken place with the bond beam has already occurred anyway. I'm just having a hard time with the idea of chipping out perfectly good, new, and for the most part intact grout for the sake of putting in polyurethane caulking all the way around. Any thoughts?

Thanks,
Keith