Re: Particleboard underlayment
The only problem with particle board is that it swells like a sponge when it gets wet, other than that, there will be no issues for it to be under a floating floor. If you are nailing the floor down, then I'd replace the P-board with plywood.
Something to take into account, as addressed by MLB, is the thickness of the hardwood, padding, vapor barrier, etc. that will be under it, then adjust your subfloor height accordingly. You don't want to have 3" gaps under your door jambs, nor do you want elevation differences when crossing hardwood transitions to other types of flooring (e.g. tile, vinyl, carpet ). Elevation differences between floorings, even as little as 1/2", can be a toe grabber, as well as uncomfortable to step on with bare feet.
So, to answer your question, as long as the P-board is in good shape, there is no need to replace it, short of finished flooring height issues.
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