My experience has been that it's not worth it to change the valves on 20 pound tanks. You can buy a new tank, with the new valve, for about the same price as the valve alone. I hadn't thought of the swap at the local grocery or Walmart. I'll probably do that with the tanks I haven't already converted.
I have some 30 pound tanks that I have already changed over, and two more that need it. I intend to buy the 20 pound tanks at Sam's Club or Walmart and remove the valves for my 30 pound tanks. This is not the ideal way to do it. The valves for the 30 pound tanks are not the same as the valves for 20 pound tanks, but for my own use they are close enough.
Also, that pair of tanks has a float-type quantity gauge with electrical connections to allow reading quantity inside the trailer. These would be expensive tanks to replace.
I have found that the LP dealers I have contacted will not sell me the valves anymore. I assume that this is because of liability issues. I bought the four original ones over a year ago and have been waiting for the tanks to run dry. We don't use too much either.
Loren