The 2003 A was available only in a “somber” silver/black color scheme. 2003 was the year for the greatest single number of changes, upgrades, improvements... call them what you will.
These changes can be found here
The 2004 model received, for all intents and purposes, the front end from a 950A.
I would tend to agree with BlitzBike regarding the value of those "improvements". They are arguably more beneficial to the street rider than the dirt rider.
It's a moot point, as unless the bike is a grey market from Canada, Mexico or elsewhere outside the US, none were imported into the US by KTM. These bikes, like the 2005 are all orange/green pearl metallic in color.
The 2005 model is pretty much a carryover of the 2004... new to us, a year old to the rest of the world.
The only major mechanical improvement for 2005 that I am aware of, and previously mentioned by BlitzBike, is the addition of a second oil jet, in all likelihood designed to provide additional cooling to the underside of the piston.
The fuel capacity was reduced by approximately one gallon, from 7.25 to 6.25, by modifying the inside surface of the tank. This was done to accomodate a rather large emissions device on the right side of the engine near the head. I have not seen this device in person, nor have I seen good photos of one. This change may have occured in 2004, but as we had no US bikes to eyeball, it's hard to say.
There are some additional changes for 2005 of arguable value such as the round folding brake pedal peg.
I have not seen close-ups of the 2006 model which has yet to make it to the US, although I understand that, much like the 2005, a small quantity will be imported. The most noticeable visual difference is the addition of the words "Dual Sport" on the tank. IMO... not exactly attractive. Are they trying to convince someone of use or intent with that reference?
Anyway, that's what I know or at least think I know. The only thing on 2004 and later that is not easily or cheaply fixable for dirt work are the soft black Bahr rims and the reduction in fuel capacity... although that last item may not be an issue for some.
I have a certain bias for the 2003 model and would suggest this year in a used bike. However, they command a premium over prior years that far exceed the typical year change value, due to the long list of improvements.
A 2002, although not equipped with some of the 2003 goodies, is still an excellent bike and not something I would sneeze at should a good one come along for a fair price.
A used 2005 would be nice; the cost of changing out the rims for serious dirt work (if they've not been done already) could be kept to a minimum by having someone like Woody's Wheel Works
simply lace on better quality rims.
The front brake and fender issues are relatively low cost, and typical suspension "correction" is a common upgrade on all Adventures... unless of course you happen to weigh 150 lbs.