I recall seeing some type of message that said that seat swapping is not possible with an infant. Perhaps it has something to do with the oxygen masks, perhaps something else, in any case, I wasn't able to change the seats either in MMB (getting the same un-informative error) and IIRC at OLCI the system just stated that seat swap isn't allowed.

The best possible experience with the infant would be to be able to raise the armrest i.e. generating large open space where the baby could lay down. Unfortunately at least in ex-BMI 320's armrests where immobile. Don't know about the newer models, with AY SH J we were able to generate such a "larger bed" in A321 so it is possible, depending on the plane&type of seats.

Top tip: keep the champagne as far as possible from the kids reach