cook100,

Firstly welcome to flyertalk.

Secondly a comment to your post. I do not understand your problem. When you book a seat with LH it is clearly stated during the booking process (even in bold) that
"Please note that a confirmed seat reservation does not legally entitle you to a specific seat on board the aircraft (e.g. seat 3A), but rather only to the selected category (e.g. aisle seat, window seat, seat with more legroom). "

Reasons may be:
- On long-haul: Change of aircraft type
- On short-haul: You might be aware that the "curtain" between C-Class and Y-class may be moved
and therefore seat reservation may have to be adapted.