Our house has some 3/4 natural red oak in it. It's is by no means throughout. It's in the kitchen a walkway and the front hall. Our house is an open concept layout, so the walkway. . .is in the open and has carpet on either side of it. That said there are some door ways where the casement molding touches the hardwood. Same with the staircase. The hardwood runs under the finished wood work where the stair meets the floor.

We are considering putting new flooring throughout.

For reasons I do not want to go into here, my wife is in love with a specific product that only comes in 1/2" thick. We also do not like the look of red oak, so adding more, sanding, and finishing is not really in the cards.

If I were to rip out the well worn red oak, and replace with 1/2" think product, it would seem to me that I'm going to have 1/4" gap under all the finish work wherever there was 3/4" hardwood before.

The only reasonable solution that I have come up with is to lay 1/4" plywood down over the entire sub-floor and then place the 1/2" thick product on top of that. Is that what everyone else would do, if stuck with a similar situation?

If so, what type of plywood would you use, and how you you recommend laying it down and securing it?

Thanks for the knowledge share.

If the old floor is fastened well you could go over the old floor.
If you use plywood for after the tear out, use CDExterior or underlayment plywood.
If you are using a MIII floor stapler to fasten the flooring, whichhas a plate for half inch, you can take the plate off and use 2 inch staples to fasten the ply down, minimum fastening schedule is 6 inches on the side of the sheets , and 8 inches in the field. One and a half inch staples are good,too. Your half inch flooring can be fastened with inch and a half.
It makes the stair tolerance better to do the removal, then plywood, since all risers heights should be within 3/8th of an inch.