Just something I read in Boston's Gun Bible. He tells the tale of buying a 16" AR he didn't know was chopped, then explained something about the distance of the gas port being effed (I guess it's closer to the receiver on a proper M-4gery?). If I recall correctly it has to be opened to compensate for that, but I won't swear to it.
Length of current barrel?
How short do you want to go?
This is pertinent information and will dictate whether or not the port needs enlargement.
I cut and pin a slew of barrels (14 in the shop now) and very rarely do we have to open up a port.
Usually the only time we run into issues is on suppressed rifles or some ding dog wants to use a carbine barrel for a pistol build.
Throw the upper in the mail and we will get it done.