Now I found that as well.

Using the mobile app, the explanation does not appear anywhere during the booking process. One can find it when clicking on the “…” on the bottom right hand corner, and then
> More
> General
> Legal and Data protection
> Conditions for seating
> (scroll down a lot)
> Are there any benefits for HON/SEN passengers
> (realise that it does not talk about the ability or not to change a seat once it’s assigned when you’re a status customer)
> (go back)
> Can I choose my seat during online check-in, as before?
> (realise that it makes a general statement that it can be done only for a fee, but does not mention any special rules that might exist for status customers)

Again, not saying that it’s not my own fault. But to anyone suggesting “read what you book”… well, I have better things to do than do forensic archeology in the bottom layers of the terms and conditions. If those people always read the terms & conditions, congrats. If the way things works had been explained more intuitively that would have been helpful, but I suppose that would require the airline to think about things from a customer perspective. Which clearly is not the business they’re in.