Assuming you can find replacement valves that have the same spacing (for the vertical pipe) as the existing ones, you can do this.
If you get a contractor to do it, he's just going to do what MrFixIt is telling you to do.
All you need is either a PVC Pipe cutter (about a $15 tool that is basically a combination of a utility knife blace and ratchet pliers) for pipes upto 1-1/4", or a simple hack saw for larger pipes and basically cut the pipes just below the existing valves. (If a hack saw is used, you might need some files and/or sand paper to smooth rough edges)
Next, you just have to get the right fittings to connect the new valve to the cut pipes, connect the fittings to the valve and glue fittings to the pipe. The "gluing" process is pretty simple... you wipe the pipe with primer, let if dry, add the glue, and connect (we get get you more exact detailed instruction, it's a process that is WAY much simpler to learn than soldering copper pipe). All it takes is a touch of practice.