Replacing old floorboards

Ply doesn't have tongue and groove joints. So not an issue if the joint is across the Joists, But it won't work where the joint is along the length of the joists. Unless you rebuild the floor joists.

I have replaced T & G boards in the past with 18 mm ply. Turned out a good job.
To do it well requires a little planning. When you lift the Floor take note of where Cables and CH Pipes run. Does any of them need to be moved ?. Do you need more Sockets in the Room ?. Think of how you might access them again if needed without taking up the whole Floor again.
Find a Square Corner of the room to start in. Or Start from the middle of the Room out to the Walls. Try and keep the Ply square with the Joists also as this will cut down on the need to cut Sheets.
Lay the Ply across Joists and cut at the middle of the Joists.
Personally I prefer to stitch a piece of 50 x 25 mm timber along the Joist where they meet for added strength. Don't forget to leave a 10mm gap around the Walls for expansion. You can cover this with the Skirting.
You'll need a good Skill Saw to begin with. A Pilot Drill bit and a Counter sinking Tool for the Screws. A Hammer and a Nail Bar.
For Safety pull the nails as you go while lifting the Floor then you don't have to worry about them. Many a person has walked on one after pulling up the boards and ended in Hospital. good luck with it.

Used to be able to get tongue and groove chipboard for exactly this job.