I own a 1924 Bungalow in Rhode Island. Lots of old and original wood work throughout the house.
The attic is a walk up with a fair amount of room with a shed dormer which made a small but manageable bathroom.
Several years ago I framed, sheet rocked and carpeted for my 3 daughters. Yeah, carpet, except in the BR. That got tile. Carpet is what the wife and kids wanted in the living space.
The original floor was 3/4" X 5 1/2" T&G pine. All in good shape. I re-secured with a several hundred sheet rock screws and covered, using screws, with 3/4" particle board and laid the carpet over that.
Daughters are gone and I ripped out the carpet and I plan to install Hickory throughout the space and rent to a friend.
All the particle board is fine as there was never any water damage.
The major question. Will I need to remove the PB and replace with plywood? Cover the PB with plywood? What size? Or can I safely nail the new flooring to the existing flooring. If so, do I need to use long nails to ensure I hit the old T&G?
Thanks for any advice.

I would definitely remove the 3/4" particle board. No advantage for leaving it and several good reasons to remove it. The main reason is that particle board has no holding power for flooring cleats, or staples. Also, leaving it gives you an extra 3/4" of floor height to have to deal with, or covering it with plywood as you mentioned, would raise it another 1/2"-3/4". No reason to do that. The easiest course of action would be to unscrew and remove it.

Check the sub floor for flatness, install a good moisture barrier for a nail down installation and you are all set to install your hickory over the original pine plank sub floor. I'd use 2" flooring cleats. Power Nail is a great brand and they're made here in the USA.

Also, for future reference, you want to use the proper sized deck screws, NOT sheet rock screws, for securing your sub floor. The shank of decking screws are designed differently to not bind, they're stronger, less likely to break or snap and they will have a torx tip head that's much easier to drive and less likely to strip than a phillips head.

Thanks for the advice! I never thought of removing it.
Yeah, I snapped a lot of binding sheet rock screws in the process. This was done 25 years ago. I was poor then and used the least expensive products that would work. And the screws were "donated" by a friend who helped me hang the rock.
My parents actually purchased the carpet. A nice berber that wore incredibly well.
Now with the wife an ex and the kids all on their own, I'm doing it my way.
Yeah, the added height would be a PITA to deal with. Didn't think of that either. Not so much with the baseboard as I am making new in my work shop, but for the low and slanted ceiling height I already have to deal with and a few other things that would need to be changed.
But in reading things on this site, I came to realize that aside from the nails not holding well in the PB I was also wondering if the PB sandwich would cause any issues with seasonal movement of the new flooring? Would the movement cause deterioration of the PB?
But regardless, it's coming out. Not looking forward to that. Sounds like a good excuse for a kegger work party!
What is the best relief paper to use under the new flooring?

PB is dimensionally stable,...unless and until it gets wet and then look out. That's why it gets used a lot in furniture and cabinet making as a substrate under veneer and laminates, that and it's much cheaper than a good plywood.

I am assuming the attic is a climate controlled space. You shouldn't have any moisture issues on the second floor, unless there's a plumbing accident or roof leak. That said, seasonal movement will likely be greater in the winter with the heating season. You can minimize that with a humidifier. You are right to remove the particle board.

As far as "relief paper" I used AllGlobe silicon vapor barrier. It's a heavy stock white paper that has a silicon coating on one side. It's cleaner to work on than roofing felt and better quality than a kraft or rosin paper. You could use a 30lb roofing felt under it if you wanted to for slightly better sound insulation but it's not necessary.

I have a Berber in my office and it does wear incredibly well. Looks good too but...I'm thinking that is one place I could go with Hickory in my house. I have a dark stained lightly distressed Ash in the living and kitchen .