Pouring slab but second story sags in old home
You need to slow down and rethink this. You are on the verge of making a couple of serious mistakes. Rather than directly answer your question, I am going to advise a plan to do this correctly.

First, you should not pour concrete around wood support posts. This is a good way to invite rot and an unstable support system.  The floors should be jacked up a bit (1/2 to 1 inch), footings poured before you pour a slab. Trim the posts or replace with steel and set them on the footings. If you keep the wood, they should be set on top of the slab/footing, not in it. 

You should determine the root cause for the second floor sagging. Determine if the posts in the basement are part of the problem or if the cause is higher in the building.  You didn't accurately describe how and where the sagging is taking place, so impossible to even start describing what can be done to correct it. 

The fact that you are even thinking of encasing the posts leads me to believe you should seek some local expertise to advise you. Don't compound your problems. Get some competent eyes on your project before proceeding.