There are a couple of problems of just laying new over old.
1) It raises the height of the floor by 3/4". This means that, now, maybe your feet won't fit under the counters anymore, or a door won't close (or will have to be 'shaved' on the bottom 3/4"), or a table leg won't sit flat. And so on. DO think ahead to imagine what will not be level or what will have a 'height' problem. YOu can use a scrap piece of 3/4" 'any-type-of-wood' to lay around and see if you'll be OK.
2) IF the particle board swells, or is swelling now, it may exert enough pressure to push up your floor so that, later, it will feel a bit wavy. This pressure is VERY strong even if you're using screws to install the 3/4" plywood over top.
IMO, I'd rip out all the particle board and install EXTERIOR grade 3/4" plywood. It has a resiliance to moisture, and such.