Far easyer and better to replace the whole panel. Each piece over laps the other, so if you cut it in the middle there's no over lap.
Your going to need what's called a cats paw nail puller to pull all the nails where the seams overlap and in the field (middle) so the panel can be removed. If all the nails are pulled you should be able drop the panel so the Z molding does not get bent and stays in place.
Measure the panels height before removing it. Years ago it was common to see 9' panels, now there special order and will cost abot 60 to $90.00 each.
T-11 was never meant to be installed within 12" of the ground any closer or if some builds a deck up againt it and it's going to rot.
Most often I suggest people just go right over it with vinyl siding so there will no longer be any maintance, just a pressure washing once in a while. Lot easer then painting.
I also suggest not using paint, instead use solid color stain. It can be mixed to almost any color to match the rest of the house now.
Someones going to suggest using oil based paint I'm sure, do not do it, oil paint does not breathe and will peel in no time. Stain soaks in and does not peel like paint will.
When you nail it back up use double dipped spriral #8 p nails.