Thank you Gentlemen for your valued input.
My concern was not bonding between the old and the new concrete.
Would you recommend doing every other post and then waiting a few days for the concrete to cure before doing the alternate posts?
I was wondering if I could, without additional excavation, get similar results by filling the gap with play sand half way up, filling it with water for compaction and after the water percs into the ground, filling up the rest with more sand. I'd be grateful for your thoughts.