Originally Posted by
Swiss Tony
A lap infant on BA pays 10% of the adult fare regardless of class of travel. This is different to the policy for AA domestic flights where they're completely free.
I've never done this in two stages as you describe, but always bought & upgraded the tickets at the same time. The options we had were to pay 10% of the club class fare, or 10% of the WT+ fare, plus the 10% in miles, or 10% of the club class miles. It worked out about the same if you value the miles around 1.7p each IIRC.
The AA policy referred to (and UA is the same) is that on international flights a lap infant pays 10% of the class flown. So if you buy economy and upgrade to business for yourself you must also pay the difference between 10% of full fare business and 10% of full fare economy for your lap infant. Supposedly they sometimes forget to collect this when you upgrade, but those are the rules. The 10% fares are not upgradeable with miles, and using miles for a sear on AA/UA is at the full rate, not 10%. I think BA is much better than all that.
UA/AA policy on domestic flights is that lap infants are free (for now).