Having been through this, perhaps I might offer some perspective on BA’s likely position:
- They will not replace the GUF because half of it (for the return journey) will still be used. You can’t have half a GUF in your account. There is no point in even asking - the law is on your side, don’t get distracted by offers or requests outside it.
- The compensation due is not 75% of the fare for the leg plus 75% of the GUF. It’s 75% of the price paid for the leg, expressed as a percentage of the total journey distance (less the usual taxes). As far as EC261 legislation is concerned, you’ve paid (probably about) £1500 and been placed in F. Your reimbursement amount is based on that £1500.
- BA will argue KL J=BA F because they are both the highest class of service available. This is easy to counter but needs some fare research on BA J fares v KL J fares v BA F fares plus other cabin comparisons.
- They may well try to claim the OP voluntarily took the downgrade. This has no basis in law, but having an IrOps note on the PNR helps enormously (I made sure it was entered on mine).