It would be helpful if we could undo the upgrades on weapons. (ie. turn them on and off). Certain upgrades, like scopes, allow you to go back and forth. However, the upgrades with a single option are permanent. I had a really great auto rifle which I loved the high fire rate on. I tried out the upgrade for it which slowed it down for increased damage and did not like the change. Unfortunately, there was nothing I could do about it.
I had the EXACT same problem with the Arconia AC3 (something like that). I had to spend a couple of hours to find the weapon again fresh without upgrades. I later just stored the slow one!
Actually you can revert the upgrades, I have several times. I'm a Hunter and hate all the grenade upgrades so I usually always go back to basics. Same with my gun, I've switched scopes before back and forth.