Pulling the post depends on what is to follow.

I had a neighbor try to pull some wood 4x4 posts which must have had 2
bags of concrete mix per hole. He wanted to replace the fence with a
brick retaining wall. Borrowed my pair of farm jacks - all it did was
break the posts off at ground level.

I recommended that he break them all off at ground level and rent an
electric jackhammer to break the base up if he really needed the
concrete removed.

If the replacement fence can offset the posts from the originals, then
breakoff is the fastest. To replace just ONE post in the line may
require removing everything.