In my research, most of what "upgradable" refers to is mostly stuff to do with the mechbox, as well as tweaking with the effect on flight of the BB. What's usually going to have to be replaced (and/or upgraded) will probably be the hop-up, the gears, and the battery. Those are things that will eventually wear down.
Now in terms of pure upgrading, people usually modify their spring (tension of various springs gives the BB greater or lower velocities), their batteries (most stock batteries have just enough mAh-milliamp hours-to power a game, so most people go with larger capacity batteries), and maybe the internal barrels if necessary.
Point is, most of the stuff worth upgrading are the internal parts. Most of the exterior is for aesthetics, and would be comparable to putting M16 hardware on a Tippman A5. Just for looks.
What's more important: morality or the freewill? In our society most people would say freewill. But we mustn't forget our morality.
Our morality guides our freewill to do the right thing.