Thanks. Your advice is very close to that of a friend of mine. But my wife just loves the old floor. I expect this is one we'll have to learn the hard way.
Interestingly, we are not filling it with anything just now. We are gonna put the kitchen back together and learn just how hard this is to deal with. We've got a lot of labor but little money in it. If we can stand the work, we might as well give it a shot.
I'm happy for now not to fill it, given your warning about the danger of the expansion and the contraction.