In the school system here we did a LOT of panelboard change outs, using existing cabinets.
They were typically 42-circuit fuse boxes in huge metal cabinets embedded in masonary walls with rigid pipes coming in top and bottom of the cabinet. We'd usually just label the wires/ckt #'s, rip all the guts out (incl. grounded conductors) and screw in a new 42-ckt SqD bolt-in panelboard, put the cover on, and make a drawing of the sheet metal that would need to be fabricated to cover the gap between the old cabinet and the new panelboard cover. Later that day, we'd take the cover back off, install the new sheet metal 'filler' plate, and screw back down the panelboard cover w/ door.
We probably did a few hundred of these (there are 110+ buildings in the school district, multiple panels per building, and most buildings were of the same vintage at that time) over the course of two (three?) summers and never failed an inspection for using this method. Not that "passing" means we must have done it right, just that everyone involved thought it was the right thing to do. I can't imagine changing out the cabinets, given the size of them, and all the rigid conduits going in the top and bottom (and sometimes sides!)