03 has AVCS, the first of the wrx's with it actually. There are no official updates for -03 to -05 wrxs.
I would say you could use an 05
ecu in an 03 car. AS long as the input/output's are the same. But whats the point?
There may be small differences in the software, but it will mainly be centred around efficiency and economy.
Kato's right. Only dealerships have access to software updates from Subaru.
Just ECUtek it, or Verso