I recently bought 2x tix Akl -LHR in Y and called up to find dates that had A availability for elite Premium upgrades.
Found dates and instantly went about booking. After confirming booking then added all upgrades online. The Akl lax and lax Akl all cleared as expected, but the LHR- LAX did not. Rang up the call centre and had a lot of discussion, it turned out after I researched a lot that it was the premium cabin tax that was the problem. It took over an hr on the phone for the staff to calculate the correct £ to nzd tax applicable and charge that to me before the upgrade could be confirmed. Got it in the end but thought it was hard work.
Doubt many others would go to these lengths. Seemed ridiculous