Thanks all. Rex Cauldwell's Wiring for Pros says that NEC allows derating a 50A range to 40A on the basis that homeowners will never have everything on on the range. So you would run #8 wire from a 40A circuit to a 50A receptacle and plug in a 50A range.
Cauldwell disagrees with NEC on this, but having tried to wire up a receptacle on #6 wire in a junction box, I can understand why this might be useful.
In my case, I am looking at a 40A dual fuel range. I was interested in up-sizing for future needs, until I tried it. Just wanted to make sure I did not have to replace the receptacle.