The reason why you can't flash the 03 is because the software won't allow it, the OS's have to be compatible.
You do have the correct SN PCM 463, that is an '05. I'm not sure if you can flash an 03 cal on an '05, I just haven't had to do it yet.
Go ahead and try it, just save your current file first, then you can go back to it if it doesn work. BUT, you may get the same error because you're still trying to flash over an 05 cal.