I don't know what climate zone you are dealing with, but if your budget allows, you are probably better off tearing it out and starting from scratch. Unless you significantly increase the width and height of the wall to accommodate at least a 4 to 6 inch depth of reinforced concrete cast as an overlay, I think it will break down over time due to weather, more so if you have freeze/thaw issues where you live.

Seconded. Take it down, that should be the easy part. Unless you are going at least 6" wider on all sides and you are sure the existing base of this wall is adequate.

Do it right the first time.

Thanks for the replies. I guess I knew that there would be no shortcuts :(

I have a flagstone deck that the grout is sinking and it all looks old and ugly Can I cover this with concrete. How thick should it be and how far out from the edge of the flagstone?

First, I really have no idea or knowledge about this, but how about stucco over the stone? I grew up in southeastern PA and there was an old stone house that had been covered with stucco. Seemed to work okay. That was fieldstone, though, so not sure if that makes a difference. Maybe Bahia and natural-sens would know.