(Reposting with some additional info based on the comments.)
People are now successfully product changing their Citi cards to become the new Custom Cash card. Readers in the comments report being able to product change their Expedia, Rewards+, Drivers Edge Charter, Diamond Preferred, Premier, Prestige, Preferred, AT&T, Dividend, and American Airlines AAdvantage cards to become a Custom Cash card.
Double Cash cards can not yet be product changed to Custom Cash, though you can try changing it over to a Rewards+ and from there to change it to a Custom Cash. Some people heard from reps that Double Cash will be able to be converted directly to Custom Cash beginning August 1, 2021. Citi Simplicity is a mixed bag with some having success product changing and others being told the August 1st date for doing that product change.
The big question remains whether your card number will change when you product change to the Custom Cash, which might depend on which card you are changing. This matters because, officially, whenever the card number changes it would be considered a ‘card closed’ to reset your 24/48 month clock, whereas if the card number remains the same it won’t reset the clock. You can search the comments below for data points on when the card number will change from a particular card, or you can ask the Citi rep and hope for the best. I’d guess that product changes from cobrands (e.g. AA to Custom Cash) will get a new card number, but product changes from within the ThankYou family (e.g. Preferred to Custom Cash) might keep the same number.
The Custom Cash card uses the ThankYou points system and allows points to also be cashed out at 1 cent per point. The card launched in June and initially was not accepting product changes, but today they began allowing them. When you product change to the card you won’t get the signup bonus, but some people prefer doing it anyone to avoid signing up for a new card. Reports are that the product change can even be done via chat.