Originally Posted by
LXFlyer
I often have issues with the LH website on non-220 tickets

Calling them works better.
This is the best advice to achieve a quick result.
The only way I have been able to book/change LH seats from a booking made on united.com is to create a Miles and More account, login and find the booking through that method, change the seats, and make sure I change my FF number to United. It's a bit of a pain but if you don't like calling up it has worked for me.
Adam