Costco Wholesale Corp., American membership-only warehouse club, named Citigroup Inc. and Visa Inc. as its new credit partners On Monday.

Costco Wholesale Corp announced Citi, the world's largest issuer of consumer credit cards, would become the exclusive issuer of Costco's co-brand credit cards and Visa will replace American Express (AmEx) as the credit card network or Costco in the United States beginning April 1, 2016.

In February, AmEx said that it had been unable to reach a deal with Costco on renewal terms. It said that the 16-year partnership in which Costco only accepted AmEx cards will expire on March 31, 2016.

After the announcement, credit-card firms teamed up with the fast-expanding wholesale club, which sells everything from car tires to smoked salmon.

However, Costco also said that the deal isn't closed yet as there are many details that must be negotiated in advance.

Costco guaranteed customers that they will receive details about how they could make the switch-over in the next few months and reassured them that the transition from AmEx to Visa cards will be as easy as opening the mail.

Costco revealed that that AmEx would still be capable of handling closed out Costco accounts and collect payments. However, experts warned that a similar move performed by Costco in Canada last year turned out to be disastrous for clients.

AmEx credit cardholders had to line up with crowds at their local Costco stores and then they waited again in another line for new photos. They finally waited for some more until the new card was mailed.

But Bob Nelson, a Costco spokesperson, said that this situation would not be repeated as Citi Group is presently negotiating with AmEx to buy its portfolio of Costco accounts. Citi will manage the credit card accounts and issue the cards, while Visa would process payments.