Since you are working with copper, and a plumber is coming out anyway, you may want to have him sweat in a coupling and raise that valve, and also put in a T with a curb cock, so you can cut off the water at that valve and drain your house back to that low point through the curb cock into a couple of buckets, in case you need to do any other work. This might sound funny to some but I have one with a hose connected inside, just in case I start a fire in my basement shop
Stop thinking/reading my mind
I had the exact same thought! I definitely want the valve a little higher off the slab...don't know why the original plumber did that way.