Is it okay if the underlayment is thicker than the subfloor? When the home was originally built it looks like they used a thinner plywood on top of the joists with a thicker underlayment on top of that. Did they reverse the order? Does it matter? Should I do the same when I lay the new floor? It would be easier to tie into their work if I copy what they did.
1/2" plywood under 3/4" underlayment is fine. The underlayment will be screwed to the subfloor at 6" on all edges and 8" on center in the middle in all directions. Use a construction adhesive between the layers to make them as one, the screws will draw it all together.