URGENT: in Milan, have web ticket, how to get SBB

Hi,
I have get ticket from Milan to Zurich for tomorrow train from SBB website , but I just found I should change it to a real ticket in Switzerland. I plan to buy a ticket to take another train from Milan to Chiasso, then change my web ticket to a real ticket at Chiasso and take my train (arrive at Chiasso at 7:52) to Zurich . My question is, when I arrive at Chiasso, it's too early, I guess there is no people in train station office, how can I change my web ticket to a real ticket?
Or, can I wait at Chiasso till the people arrive office then change my ticket, but maybe itàs latter than my train arrive Chinasso(I mean latter than 7:52)?
Thank you so much. Very urgent.
BR

Hi BR,

I'm not sure why you should change your ticket. I would advice to ask the train personnel whether this is really necessary before spending time on that. The ticket office in Chiasso will open at 9:30 (Sundays). Further opening hours are on the SBB website.