CMMG 9mm BCG/Barrel Ejection

Hi all,

Does a standard AR15 upper receiver need any modifications to run the 9mm barrel and BCG?

I know the website says that it needs a "modified ejection port", but does it need anything else to cycle? I'm trying to get a budget estimate before I end up sunk-cost-fallacying myself into a $1,200 "spare-parts" "budget" build