I have more than my share of experience installing vinyl composition tiles, but I have never installed plastic laminate flooring. That's why when Bud Cline insists that there will be a problem with the installation of the laminate flooring if it's installed over a plank subfloor, I can't see any reason for there to be a problem, but if it were my house I would take the time to check on this with someone more knowledgable than Bud, like the Installations Manager at your local flooring store that sells the kind of laminate flooring you want to install. Their installers would have install lots of this laminate and if there were any problem associated with installing it over a plank subfloor, the Installations Manager would be aware of the potential problem in doing that. So, just to be on the safe side, I think you should phone any of your local carpet retailers that sell plastic laminate and talk to their Installations Manager to see if he is aware of any known problems installing laminate flooring over what you will have; namely 1/6 diagonal planking for the subfloor and 19/32 inch plywood for the underlayment. If that Installations Manager is not aware of the problems that Bud is warning you about, then ignore Bud.
Now, for the problem at hand, how much of a discrepancy in height is there between your plywood panels? Are we talking 1/16 inch, 1/8 inch, 1/4 inch?
You see, the problem here is that if the 5/8 inch particle board nailed down flat on your plank subfloor years ago, then why would there be a problem nailing your 5/8(almost) plywood down flat on those same planks. (if what Bud is saying is true, and that the boards are warped and twisted, then they'd all be warped and twisted, and the plywood we put on top of those warped and twisted boards would still line up well at the edges and corners).
I think what you may be encountering is swelling of the particle board underlayment you have. That is, your existing particle board may be quite uneven. You say you had a carpet over that particle board before. Well, if at any time during that old flooring's history, someone overwatered a floor plant or spilled a glass of milk, or knocked over an aquarium, and the excess moisture dripped down through the carpet, through the underpad and got that particle board wet, then it would have swollen (perhaps a little, perhaps a lot, depending on how wet it got) and the result may be that the top surface of your particle board is wavy as a result of swelling. So, when you set sheets of plywood over that particle board, those sheets wouldn't be expected to meet up well at the corners if the particle board under it is wavy. But, that problem would be corrected as soon as you pull out the particle board and put that plywood over the plank flooring, which appears to me, at least, to be laying flat and level.
It may be a good idea to take a bright flashlight and shine it along your floor. Even a bright light held close to the floor would exagerate the roughness of the floor so that you could see if it was flat or not. That would give you an indication of whether or not the existing particle board is flat enough to do the test you're doing. I expect you'll find that your particle board is swollen in different places, and so laying flat plywood over it doesn't really indicate if you'll have any problems installing the plywood or not.
Unfortunately, that's one of the characteristics of particle board that makes it a lousy underlayment for flooring; if it gets wet, it swells up and looses both it's strength and hardness. And, if my explanation as to why your plywood panels aren't meeting well at the joints seems reasonable to you, then I'm sure you'd agree that your existing particle board wouldn't make a very good surface over which to install your laminate. That's cuz the plastic laminate "boards" won't be supported along their whole length, but would have a bumpy substrate under them.
Now, if it turns out that you pull off your particle board underlayment and your plywood panels still don't meet well at the joints and corners, then about all anyone could do in that situation is to use something called a "cement based floor leveling compound" like Mapei Planipatch to "float" the floor.
Essentially, what Mapei Planipatch is is a cement powder that can be mixed with water to form a slurry. You simply spread that slurry over either the whole floor or just over the joint areas to help smooth out abrupt changes in the floor's surface.
floor patch | MAPEI
Mapei is the largest flooring cement manufacturer in the World, and Planipatch is their leading floor leveling product. I will be able to explain to you how to use the stuff to level out the surface of the plywood as best you can. Which won't be very good because we're fundamentally relying on the sleeping 2X4's and plank subfloor to be reasonably flat. If they aren't, then any plywood we put on top won't lay flat either.
So, if things aren't flat underneath your particle board, then we can expect problems with the plywood as well. I suspect everything under the particle board is reasonably flat. My hunch is that the particle board has been wetted in different areas by different amounts over it's history, and isn't flat any more.
But, for the time being, phone the Installations Manager at your local carpet retail outlets and see if any of them have experienced any problems:
#1. installing plywood underlayment over 1X6 diagonal plank subflooring, and
#2. installing plastic laminate flooring over #1.
Finally, the manufacturers of tongue and groove plywood subflooring recommend that a 1/8 inch gap be left between 4X8 panels of their subflooring when it's being nailed down to floor joists. That is to allow the T&G plywood subfloor panels room for expansion as they absorb moisure from the air after installation. I have never heard of a similar expansion space being recommended for plywood underlayment. I would ask the Installation Managers at your local carpet retailers whether you should would provide that 1/8 inch expansion gap around your 19/32 plywood underlayment panels or not. If it was me, I would provide that space because doing so won't do any harm. The laminate flooring is plenty strong enough to span such a narrow gap, and in future if you ever want to replace the laminate with a thinner / softer flooring, like sheet vinyl, you can always fill those gaps in with a product like Planipatch.