I've noticed other situations where small ships, like AVDs or DDs, will not upgrade at smaller ports even if the SY requirement for the upgrade is 0.
So I look in the manual:
ORIGINAL: Manual page 238
When this value is "0", upgrades/conversions can be made at Ports with a sufficient "ability" rating, as explained in the Port Section, below.
Trouble is, I can't find anywhere in the "port section" (such as it is) that explains port abilities as it relates to upgrades/conversions - only repairs. Is there somewhere that explains this relationship? The only thing I can find is at the very end where it mentions that (Damage * 10) + (Ship Tonnage / 500) is the "Port Assist Cost" for stood down ships (presumably would apply to upgrades), or (Damage * 20) + (Ship Tonnage / 500) for readiness mode. These values are taken from section 14.2.3.8.1 on page 248.
North Carolina is listed as 37,484 tons. So it would require 74 or 75 Port Assist for repairs. A level 6 port only provides 63 (per table on page 239). Can someone confirm that this is used for upgrades?