Part of my downvote is because I see this request as two different things. The first is changing between identical barrels to stop overheating, while the other is the option of changing the characteristics of each weapon. Splitting the request would make more sense to me, but not you perhaps :)
The reason not to add overheating barrels is a matter of priority for me. Even though it is perfectly realistic to include, I would rather see Bohemia focus on other things.
If this was to be implemented correctly, it should be possible to for an assistant gunner to change the barrel, as he would probably be carrying the spare. And then the AI should be able to do this as well. I think this would take more development time that I would like spent on other things. However if this suggestion was separated from the other, I would certainly not downvote it, and perhaps upvote it depending on how you suggest it is implemented.
As for the changing barrels for customization, for me using development time for a feature that has poor value, from my perspective, is wasteful. The difference between having two different rifles, or one rifle and two barrels, in ammo box is negligible. And the ability to carry all the different barrels, receivers, etc. around with you to change weapon type in the field would only enhance the one-man-army tendency on public servers