Nail the new floor over the old floor?

Q: I have 2 1/4” wide birch hardwood floor and I would like to replace it with 3 1/4” oak hardwood. Can I nail / install new floor over old floor, or would I have to remove it and nail down a 3/8” plywood sub floor?

A: It is theoretically possible to install directly on top of the birch provided it is well secured. It seems such a waste of a solid 3/4 thick floor though. Also, the birch needs to be fairly flat. You would probably be safer installing the oak floor on an angle rather than in the same direction. If it was my floor, I would try and salvage the birch and install it someplace else or find someone who reclaims wood.