Porches so often become a waste dump for the contractors working in the house. This waste has a tendency to settle and decompose, leaving the floor to settele.
After you have removed the concrete, make sure there are no materials in the fill that will decompose and settle over the years. The fill should be thorughly compacted dirt up to 4" below the concrete. The last 4" of fill should be crushed limestone (#6 stone, DGA, or #54 stone).
If the porch is abutting a brick wall you need to break out a brick every 4' and place a 16" #4 reinforcing bar in the hole, extending out into the concrete. Allow the concrete to fill the hole for a key way to hold the back edge of the floor. Place heavy building paper over the holes in the concrete block foundation to hold the concrete from filling all the blocks. Use 6x6 10/10 reinforcing wire at mid point height in the concrete.
We would appreciate pictures before and after to see how it turned out.