Waaaaait, you can't just use a whole ".upgrade" file with download.c... download.c is a fairly raw "rewrite this area of flash" tool, but an upgrade file contains several different chunks of data, some of which are meant to be written into blocks of the flash, some of which are meant to be written to disk. An upgrade file is bigger than the whole of the flash chip.

The command you want is called "upgclient", though I'm not sure where to find it these days.

Peter