Drywall thickness for ceilings
I would replace the ceiling drywall with 5/8&quot; material because it is much better suited for your ceiling joist spacing of 24&quot; than would be 1/2&quot; material.
There is no real reason that the ceiling drywall has to extend out to be over the drywall that is on the walls. The wall material can help support the ceiling material if it does extend out over the wall drywall. But if your ceiling joists have backing lumber between the joists at the ends and there is joist exposure along the walls parallel to the joists that can have screws driven through the 5/8&quot; ceiling drywall into the joists and backing lumber then the only real need is to bring the new drywall up close to the wall material.
New taping joints along the ceiling to wall corners should easily make for a clean finished corner despite not having the ceiling drywall tuck over the wall drywall.