The Octane and Octane2 are basically identical systems. Some changes were made with the CPU board (the newer revision supports R14k CPU's and enhanced the maximum Ram from 2GB to 8GB), the XBOW frontplate (where the CPU board, XIO slots etc. are connected; the newer revision 1.3 support V6/V8 graphics; 1.4 supports V10/V12), the graphics subsystem (VPro replaces SI/SSI/MXI, SE/SSE/MXE), the power supply and the body (blue instead of teal (green), new logo and model type font). A mixture of different parts is possible (like R14k with SI graphics).
To upgrade an Octane just replace at least the CPU board incl. CPU. The rest is optional, although an upgrade to VPro graphics needs a newer XBOW and maybe the newer power supply (XBOW >=1.4, PSU 060-0035-XXX, IRIX >=6.5.10, PROM >=4.0 for V6/V8, >=4.5 for V10/V12).
The Body can be replaced alone, of course, but that would be a fraud ;) Nevertheless a complete upgrade should contain the body for completeness.