Lufthansa changing the terms of the sale post-purchase
I booked a Lufthansa flight with a rate that allows for unlimited rerouting and rebooking without penalty. I'd only have to pay the fare difference.
The first two times I rebooked, there were no issues, the options provided where the same I was seeing online. Now, I need to rebook again and I can't do so online. When I called, I was told that they intentionally blocked this option because I had already used it (I have no idea why that would make sense). I was also told that they are intentionally not offering me the "cheap" flights. They want to charge me over 600EUR to change the flight, which is the same price as booking a new one of the cheap flights.
I was told that this is an internal policy and that it's not stated as a condition anywhere, but that they don't offer all options for rebooking.
The rep I spoke to did not give me any reasons at all regarding why some flights were not available for rebooking. She couldn't even share the criteria they were using. This means that to find a flight that isn't insanely expensive, I have to ask via phone for each and every day within the window of time that suits my needs.
Is this even legal?