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I called Delta Rep 2 times today, being told 2 different answers:

Answer 1: From Delta Singapore Call Center Chinese line
Linked reservations request complementary upgrade only applys to US 48 Continental States. Alaska, Hiwaii, Puerto Rico not included. So the lady says she cannot click the "First Class upgrade request " for my sister.

Answer 2: US Platinum Line in English
Linked reservations request complementary upgrade applys to US domestic flight, Mexico, Canda, Caribbean Sea，Central America. Basically except for long hual international flights. She believes I should able to click the "First Class upgrade request " for my sister. However, she cannot do that as well. She put me on hold for 20 mins, and asked the other department. And came back telling me , that is the IT error, and she cannot do anything at this monent. She suggested me to call back later. Or on my next flight, go the the Skyclub Agent ask them for help.

Totally 2 different answer. I am confused now. Lol.