just to update on the outcome: phoned in (surprisedly no wait at all on the blue line USA) and CS was able to move the flight to the same day of my LHR-LAX. Did not budge one bit regarding waving the $55 fee (rightly so), it was a firm no. I just wished I called up within 24 hours of booking instead of getting lost in too much panettone.

Side note, he was also able to make the same change to my husband's reservation, who is traveling on the same flight with our other child, but had to be on a separate PNR in order to use his own 2-for-1 voucher.