Originally posted by robandgill
Not being an expert on this, I suspect that with an X class ticket, if you upgrade at the airport you will be asked to pay the cash difference between an X fare and upgradeable economy fare (Y,B,L,M) before then upgrading (cash or miles) to a PE fare.
There is no guarantee though that there will be availability at the airport when you get there.
Bingo [y]
An X ticket is not upgradeable in advance of the day of travel, using miles.
What you've been offered is one of two things:
1) An "airport special". Subject to availability, the airport can offer discounted (cash) upgrades to the next cabin (at around 200-400 quid each way, depending on the day) that can be purchased at the airport. The price is completely irrespective of the fare code you paid for - it's the same for everyone.
2) To pay the difference between an X and an L/B/Y. This will run at around 50-250 quid depending on the season and availability of the next class (not sure if they try and apply any change fee to do this). They will then allow you to upgrade with miles from that point at 10k per sector. The original amount you paid, however, remains non refundable (so it doesn't buy you a strictly flexible ticket)
(Already in the US, so no CH blue drinks for me...)