Bought this as my first AR-15. The price was right and didn't want to invest $1k+ in a rifle i wouldn't like. Now that I have it, I like it and want to put some upgrades into it (magpul stock, rail system, ect.). Since it is a carbon lower I'm guessing if i want to do anything with that i'd have to get a new lower for it. Will my existing upper fit on a new lower or would I have to pretty much get an upper as well? I'm new to the AR and may as well be an idiot when it comes to upgrading anything on it. How much of a pain is it to change out the stock and handguard? I'll most likely get a second AR in the future but i figure since i have this one now, why not trick it out a bit... Any help is appreciated.