Bonus Room subfloor
You are on the right track. Thats and easy way to go since you are starting with an unfinished space. Apply the new sheathing in the opposite direction of the existing sheathing, making sure that no seams overlap. for a really nice job, spend the extra money and get Advantech sheathing. How will you deal with the transition into this room where you will have a small step up onto the new sheathing?