Okay...I've got a preban Bushmaster carbine.  If I want to also use this in 9mm, what do I need to get beside the upper??  I see that RRA offers complete upper with bolt assembly...what else is needed to go from .223 to 9mm??  How difficult is it for someone to make this change...is it simply swapping out the uppers?

Thanks!!

A different hammer and buffer may be needed and a mag block would also be required.

For sure you will need a mag block...this goes into your lower just like a magazine and then you tighten it with an allen wrench...very easy to install and remove...
9mm works fine with a regular buffer almost all of the time so I wouldn't change anything there until I found I had to.  If you do switch to a 9mm buffer and you have an A1 or A2 stock be sure to get a spacer (the 9mm buffer is shorter than the stock buffer)...otherwise you'll smash the carrier on the bolt against the opening on the buffer tube.  
Chances are you'll need to change the hammer...if you have the bushmaster AR-15 type hammer it will not work with 9mm (it must be modified)..I think Both Donnie-SABLECO and PeteinNH are selling hammers for around $15 so it is not a big expense item....the good thing is that the 9mm hammer will work with both calibers...

Sounds like a lot but it is not....  Have fun

Does anyone here have first-hand experience with the RRA 9mm upper?  How does it compare to the Oly?

I thought the RRA 9mm upper kit came with a mag block.