My 6x6 support post for the deck & porch range from 4 to 10 feet height. Some of the post have concrete poured around them and are starting to rott. My plan is to, support the deck, cut off the post, from 3-6 inches above the concrete. Remove as much rotten wood as possible from the concrete, pour new concrete into the hole or form a extended post up to the bottom of the freshly cut post.

My question and concern is:
1)I am not sure how much the attached post will move so I can clean out the hole and pour new concrete. But also how tight of a fit I can get between the newly poured pier and the post. Is there a trick to solving this probable gap issue? Should I use some kind of a bracket to the bottom of the post and position the bracket and post in place while the concrete is still wet?