I seen it a number of times wher they will leave the old panel, splice in it, screw it shut, and nipple to the new one next to it. I'm not saying you can't do it. I think it looks much cleaner to remove the old and fight every pipe with wires in the new panel.
More of a reason to reuse the old panel is if it is in a finished wall like a closet and the new service is across the place.
Reusing the old panels I see EC write on them "splice box, no fuses inside". Instead of screwing the cover shout couldn't you just load it up with breaker fillers if it's a more modern panel?
Tom
