The 1/2" particle board should be fine like it is, provided it is securely fastened down with no high or low spots, bubbles, etc. The new floor you put down will not be held in place by the particle board, but by the subfloor or floor joists underneath it. I am assuming your new 2.5" hardwood is tongue and groove. They use a special nail (spiral shank or ring shank) that are also usually resin dipped to keep them from working out. The only change you may have to make is use a 1/2" longer nail in the flooring nail gun.