[Arran]

Without seeing any pictures it's hard to say whether you would have to take the cabinet apart or not. I'm not a fan of paper backed veneer as the actual layer of veneer they use is ridiculously thin and the selection of cuts and species of veneer is rather bland, paper backed veneer is usually more expensive then the proper kind as well. Since this is a cathedral style cabinet if you are going to do it do it right, there is no sense in cutting corners just to get it back together. If it were be I would pick up some framing lumber like 2X6s, 2X8s, or 2X10s, and make a form for the arch with a band saw, plywood would work for this as well, it's sort of hard to describe but you would basically cut an inner and an outter form and clamp the layers of veneer inbetween. If you aren't sure what to do I would see if Steve Davis can repair the cabinet for you, or maybe make the arch at least, he reproduces Philco cathedral cabinets and parts.
Regards
Arran