Reading FAQs from Citi web page. Quoted below.
Q1. When do I start sending my payments to Citi instead of American Express?
A1: When you receive a statement from American Express please send your payment to American Express by mail, phone, or online. When you receive a statement from Citi, please send your payment to the address in your statement or make a payment online once you register for Citi® Online.
Q2. If I have a balance on my Costco card from American Express, will it transfer over to my new Costco Anywhere Visa® Card by Citi?
A2: Any balance on your Costco card from American Express account will transfer automatically to your new Costco Anywhere Card on June 20, 2016.
Now what if I have balance left over from Amex on June 20, 2016, to whom should I pay: Amex (according to A1), or Citi (implied by A2).
(Update 06-22-2016) Gave it a bit more thought, decided to keep the pre-scheduled full payment to the original Amex Costco, while adding a minimum payment to the new Citi Visa Costco. This way I would avoid the penalty if the Amex => Citi did not go through.
(Update 06-24-2016) So out of curiosity I actually posted this question to Citi via secured message. Here are the reply from them.
We appreciate the opportunity to assist you.
We understand your concern. During this time of transition, there may be
a delay in some information, such as recent payments, appearing in your
Account Online information. Be assured that the date of your payment to
American Express will be honored on your Costco Anywhere Visa account
even though you are not yet able to view it online. Please continue to
check your Account Online for updated account information as we move
through the conversion process. We appreciate your patience and
Our Customer Service number is 1-855-378-6467. If you are outside the
U.S., you may call us by dialing the operator from a land line telephone
and requesting to place a collect call to the United States at
904-954-1175. Our telecommunication number for our hearing impaired
customers is 1-866-210-0617.
If you have further questions or if we can be of further assistance,
please contact us.
Subject: Balance carry over from Amex Costco
Date: June 20, 2016 11:39 EST
Dear Sir, I have some balance carried over from Amex Costco card, $xxx.xx to be exact, on today June 20, 2016. I have not paid Amex yet, but scheduled an online payment on June 27, the due date. Will it automatically applied to the new Citi Costco card? Should I cancel the payment to Amex Costco card, and make a new payment to Citi Costco card? Please let me know. Thanks. –Mxxxxx X