Landscape contractor here. They're super easy to remove with a farm jack. All you need is some transport chain and a farm jack (Harbor Freight). You can pop a post out that's been mounted in concrete in about 2 minutes.

Take the farm jack, put it on a smooth surface, then wrap the chain around the post a few times and around the jack. When you lift the jack up, the chain will tighten and bite into the post. It'll rip it out of the ground with the concrete.

I did this recently to about 20 posts with the links, and then replaced it with a new fence with aluminum 4x4s and wooden slants. Worked perfectly. Removing the old fence was actually the fastest part of the process.