do I need to rip off the glued down underlay and then buy some more, glue it down and then re-lay the carpet over? Also what's the best way to pull up a board or two?
You might be able to run a Stanley blade, along the groves between boards, to lift boards complete with underlay.
Floorboards are tongue and groove edges, so they interlock. The only way to get one up, that has not been up before, is to cut the tongues off, at either side of each floor board. A multi-tool, with a half-moon shaped blade, makes that part easier.
Before that, you need to decide, which board would be easiest to lift, easiest, is one that has board finish, in a handy place for easing up, otherwise, you will have to saw one across the grain. Any board cut across the grain, should be cut in the middle of a joist, but avoiding the nails.
To actually lift the nailed down boards, hammer a bolster chisel down the gap, which you cut earlier with a multi-tool blade, and prise the board up.
You will likely need a couple of boards up, side by side, to be able to see very much.