Sorry, for clarity, when you go to MMB and press the "seating" link, then there are two things that can be presented to you. If you are outside of the OLCI period then you get a "view or change seats for this flight" link, whereas if you are inside the OLCI period, this option is replaced with "check in now". So you have to go through the OLCI process to change the seat, you no longer have an independent seat selection ability.

So my problem happens when I try to change the seat within the OLCI process.