If I buy a new iPhone X in November with the iPhone upgrade program, it will only be 10/12 installments paid by next September. If Apple releases a new phone in September 2018, how will I upgrade with less than 50% paid?
How is 10 of 12 installments less than 50% paid? It's 83% of your commitment.
As has been mentioned, you can pay the extra 2 months, or you can just wait an extra 2 months to upgrade. If they come out with some new fancy feature, like heat signature recognition or DNA analysis ID for the next phone, there might be a 2 month delay anyway, like this year.