If the floor joists are lower under the walls, and joists are 24" apart , the walls are carrying too much weight for the 24" joists can support. I am not sure , it is hard to tell with a post . I dont do all that many hardwood floors anymore , since the laminated stuff became the craze. There are some things you need to make sure of.. If the joists can support the weight that the wall is bearing, it may be settled after all those years. It is very hard to prop a joist up from underneath if it is low and holding weight. That would mean you would have to raise the joist, the wall , the ceiling joist
and rafters.. If the floor is lower just around the walls, it seems they settled. If they are done settling, you can just put a 4" strip of plywood around the where the subfloor is low , screw and glue it and you should be good. Again, i am not sure exactly what the problem is so I am guessing a little. If it helps, it is rare to take out carpet to install hardwood floor and not have repairs. We always have a few low and high joists. If it is only 1/8", we dont care, but 1/2" is not good. Pictures might help.
Sorry, I made a mistake. It's about 1/4" variance instead of 1/2". I went back and corrected my post. It's late and I wasn't thinking through what I was typing. Not that it's great, but it's not quite as bad as 1/2".
