It appears that you cannot switch a purchase made with the Apple Card to Apple Card Monthly Installments (ACMI) after the purchase has been completed. To receive the 0% APR installment plan, you must select ACMI as your payment option at the time of checkout. If you choose to pay for your eligible Apple purchase all at once with your Apple Card, that purchase will be subject to the standard purchase APR assigned to your Apple Card and cannot be changed to ACMI later.
Here are the key points from the website's information:
1- Select ACMI at Checkout: To receive 0% APR installments, you need to choose Apple Card Monthly Installments as your payment option when you check out. If you pay for your purchase in full with the Apple Card, it will be subject to the standard APR, not the 0% APR of ACMI.
2- Carrier Requirement for iPhone: When buying an iPhone with ACMI, you need to choose AT&T, T-Mobile, or Verizon as your carrier at checkout.
3- Ineligible Post-Purchase Conversion: The information does not mention any possibility of converting a full-price purchase to ACMI after the transaction is complete.
4- Separate Installments for AppleCare: If you buy AppleCare at the same time as an eligible product with ACMI, a separate installment plan is created. If AppleCare is purchased separately, it must be paid in full.
Therefore, if you want to use the ACMI plan for your new iPhone 15 Pro Max, you must select this option at the time of purchase. It would be advisable to ensure you select ACMI during checkout to benefit from the 0% APR installment plan.
I don't think Apple will let you get away with that.