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I'm likely to upgrade my gold card to the plat to take advantage of a upgrade offer and more importantly to utilize a bunch of benefits for upcoming travel. One of the questions I have is the timing of the benefits. For example, there is a monthly uber credit. Usually get that at the start of the month. If I upgrade now will I get the credit for this month or not until the start of next month? Same with the entertainment credit.
Also if I have purchases in bonus categories with my gold card will I still get the point earning with the gold card or will those purchases convert to the point award rate for the platnium card?
The Uber credit posts to your Uber account rather than being a statement credit; it's likely that it would post very soon after a product change. Those expire at the end of the month, so depending on when you do the PC you may not get it or have much time to use it the first month.
Monthly/semi-annual/annual statement credits should be available immediately after the product change. Rewards multipliers would be factored by the type of account you had at the time of the charge. (i.e. if you PC to Platinum but use your physical Gold card to make a restaurant purchase, you would receive 1X not 4X).