I changed my ticket at KIX airport and it was quite easy. Those with chip passports can change it themselves at the machine, but since I have an old passport, I needed to go to the JR office (on a different floor within the airport) to ask for assistance from the staff. Please note that the validity period is calculated by day, meaning if you change it on the morning or evening of the 1st, the pass will only be valid until the end of the 3rd (for example, my pass in the picture is valid from the 27th to the 29th).