Yes, they are swappable.
Same P/N for the backplane (the big board with the PCI-e slots and SATA connectors): 661-4996 in either case. Firmware is the same across the board.
There are three things that differ between the Quad's and Octo's:
1) Heatsinks
The Quad uses P/N 076-1338.
The Octo uses P/Ns 076-1329 (Primary proc) and 076-1330 (Secondary proc). These two heatsinks required for Octo operation are NOT interchangeable.
2) The actual processor board
Quad is P/N 661-4999
Octo is P/N 661-4998
3) Processors
Quad has P/Ns 661-5096 (2.66ghz), 661-5097 (2.93ghz)
Octo has P/Ns 661-4997 (2.26ghz), 661-5046 (2.66ghz), 661-5047 (2.93ghz)
The metal support plate that everything bolts to is the same, P/N 076-1344. RAM is the same on either model.
If you're going to "swap", more then likely, you'd need to somehow order a new pair of heatsinks (remember, the Octo heatsinks are proc specific), a new processor board, and new processors. The actual tech work isn't too difficult- the biggest problem is dealing with those @#$#ing LGA sockets. The Quad has the standard metal-locking "enclosure" around the LGA socket- you put the processor in and then close the socket retention top and use the lever to lock it all up- the Octo has none of these and relies explicitly on **PROPER** heatsink mounting to keep the processors good and tight in the LGA socket (so if you don't screw them down properly, you could fry the board).
-SC