What do you mean there are no issues with height differences? There will have to be if you're laying wood on wood.
Well, at least you'll feel taller.
I think he is saying the flooring is the floor-no strip over ply like what is common of more recent homes. If that is correct, he may be asking if it's ok to install new 3/4" over existing 3/4" (or ~5/8" if it's been sanded) without doing any other prep. Can't remove the original flooring in a case like this-the walls are built atop of it.old wood flooring now with no plywood underneath