Ripped out the old drywall because it was nailed right against the studs while the drywall on the other side of the stairs was backed with 1/4 1x2's. Don't ask me why but I needed to do the same thing in order for the drywall to join correctly. I also sawed out the old second floor landing and made a 50'x50' header for where the new stairs would go. I also put 2x8 joists in where the stairs were and laid 3/4'' sub-flooring in on top. I decided to not replace the post because it was way too much work and money. Instead I drywalled it.