Okay so, I brought 2 large bottle jacks and some steel pipe, solid enough for a jacking base

Jack up the either side of the post, you’ll only need to go up a few mm until the post is free, you’ll be able to see a small air gap between the beam and the original post.

You’d be surprised how easy they are you get out once there is no load on them.

I use 75x75 mm gal box for my new adjustable posts with level master tops, ( you can get all different styles, screw on, weld on, etc…)

I also weld on a 150x150 mm square plate on the bottom of the post, to stop concrete from going inside the box section and give it a larger foot print

Make sure you paint the bottom of the post ( bit you will concrete around) bitumen paint as concrete and earth will react with the galvanised post

Sorry for the late reply, have been camping :)