Reporting back...

I've upgraded IST-ACC. It took a 23 minute call to a Turkish number that was punctuated with people randomly forwarding my call to other people, at least two that spoke no English at all, one person who said they couldn't change any of my booking without the travel agent who booked it calling, and one person who managed to upgrade me but cut me off halfway through the procedure (and I only later discovered he'd managed to do it by looking at my booking online). It isn't 15,000 miles either: I can't actually tell you how much it is, but it's more than 17,000 by the sound of things.

LH4116 makes a good point: but I have over 450,000 Avios waiting to be used, and very rarely find the time to make award redemptions (and almost never manage to get the flight dates I need either). I am unlikely to ever use them. Upgrades are by far the most attractive use of these miles - I run a *G account for lounge access (and luggage allowances) only.