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Originally Posted by
yinshiyi
.... citi system will somehow CATCH me doing that and then I will lose those Premier points even though they already were transferred/merged into another
This is correct, but it's not that they will "catch" you, but rather than their system always tracks the original source of all the points in our accounts. The ONLY way to prevent points from expiring is to downgrade rather than close an account which has a points balance.
Note that if you do not have a Premier or Prestige card you cannot Transfer points to airline or hotel programs (with one unfavorable exception). If you are not planning to Transfer, why do you want to accumulate points rather than cashing out before closing?
Note also that Custom Cash has one limitation, it is excluded from the Point Sharing feature. Read more here to decide if this matters to you:
ThankYou Point Sharing capped at 100,000 points per calendar year.