Thanks for the explanation and tips. For my incoming flight, the whole trip is upgradeable (I had chosen dates because of that - I see the website now shows "miles upgrade available" when you are searching for flights). But for the outgoing, only the first leg is upgradeable now (for 46800 miles) as you suggested often happens but the second (for 70200 miles - same as upgrading the entire trip) is not, so the consultant I spoke to via the online chat suggested that I set a reminder for the second leg and if it becomes available, I should contact them to do the full upgrade for me. Problem is I don't have enough miles (yet)! <Sigh> This has made travelling on Emirates to the US a lot less attractive if upgrading the DXB to USA bit is going to cost so many miles.

(PS I find the Emirates online chat very helpful - you get a human being almost mmediately as opposed to fighting with a bot before they'll let you talk to a human.)