I’ve had this happen with LX/LH on A tickets where they cancelled a segment of mine and I was forced to change. They made a change that didn’t suit me, and when I tried to get something suitable they claimed no A inventory even though they were selling it for new bookings.

The scheme here is differentiating inventory between existing tickets and new ones… they already have your money on the existing ticket, and even though fare rules say no penalty to change, they set up their system that to change you need to pay up to the top fare even when they’re still selling the discounted F or A to a new customer. Nice agents have worked around this for me particularly when there has been a schedule change or cancellation, but I do think it falls outside fair trade to treat a customer who has already bought a flexible ticket differently (and worse than) a new buyer of the same type of ticket just because they’ve already got your money…