Originally Posted by Lux Flyer

So it's two different things. Award fares have flat rate pricing for lap infant based on IATA areas travelled/cabin. Upgrades don't fall under the flat rate award pricing, but still use 10% of the current day fare which is probably why you were quoted the $1500 amount, because it's no longer pricing based on you being on an award fare but due to being an international upgrade.

Truthfully, it's probably a grey area that wasn't accounted for in the policy since upgraded international awards are a relatively rare thing to encounter.