Apple provides details on the program here:
Clause 2.b is as follows:
Apply for and enroll in a new iPhone Upgrade Program prior to the expiration date of your Installment Loan. This includes applying for and entering into a new 24-month 0% APR installment loan (“New Installment Loan”) with Bank. Applying for a New Installment Loan requires a new credit check.
So if you intend to upgrade every 6 to 13 months, you will have a series of financed loans, with known good payments in time on the first, with a presumably hard credit check each upgrade event. Only when you make the first payment on the second loan is the first paid in full by Apple.
You’d need a credit expert to translate this into whether this is good to have a string of successfully paid off loans or the two hard checks is bad. As I mentioned in my comment, it’s not a given that anyone with good enough credit to get the first loan would benefit or be harmed by the second contract being issued before the first is satisfied. (Or vice versa if someone where this would matter would be cleared for the first loan.)