Purchased a Bushmaster AR-15 with an A2 upper. I would like to swap it for an A4 upper. A complete upper receiver is close to what I paid for the whole rifle so I wanted to just change the upper receiver assembly. Would a Rock River Arms A4 upper work with the Bushmaster lower? The RRA upper has the forward assist and port door already installed. Also, any idea what a gunsmith would generally charge to swap the parts from the A2 to the A4? I know it would vary from gunsmith to gunsmith, just looking for a general price if anyone has any experience. Thank you in advance.