Your rifle is sorta like mine before I modified it(mine had a fixed stock). As far as replacing the upper yourself, I say since you want to replace the barrel also just get a new complete upper. Although AR's are known for thier simplicity in building, I would recomend you bypass the tooling etc unless you plan on building a few. If you want to replace the entire upper with a flat top, its as simple as popping out the takedown and pivot pins and replacing the upper(Im sure you can offset some of the price of your new upper by selling the old one here on the EE). Bushmaster is considered one of the top tier weapons with colt, rock river arms, and others, so rest assured that your weapon should give you years of great service with propper maintenance and cleaning.