A few thoughts for your consideration:
If one or two valves appear to need changing then the odds are that other valves are on their way out too. If you want an even response between bellows push and pull on a key, then both valves relating to that key need to be replaced together. If you want even responses across the chords and octaves then all valves need changing together. By the time you have removed a couple of valves, cleaned the old glue off the reedpan you have already created a reed pan cleaning need. so you may as well do the rest all together. If you then decide to buy yet more valves another day, they will be from a different skin and may have different characteristics.
In short, to me it is not good practise to start changing valves piecemeal. It's also false economy. The only time that I do this is if I am providing a 'breakdown service at a playing event and something goes awry, then I usually change two valves, unless I can see say a valve with a catch on a chamber wall, or some other very specific other problem.
Picking up Wolf's concerns about valve grade, valves have a 'number' sizing system. smallest being 1, and the largest being 6. Each number size is linked to a thickness or skive range (limited skive range.) for the valve. The trick is often the choice of the leather and any competent supplier/ restorer will know what to use, and it's thickness for a specific size.
If I were Mike, I would want to know what is making me target just a very few valves, what is wrong with those valves that is causing them to malfunction. I would also take a view on the age of the valve set, then decide what to do. If it is a matter of faulty valve installation, and all the valves a fairly new, say a year or so old, then maybe change just a few valves, If the fault is poor installation and the valves are probably say 3 or 4 years old, then change both valves under a single key. If in doubt, I would suggest he change the lot
Hope this helps