You don't need to be in the upgrade program. Roll your own with Apple Card interest free payments and get 3% cash back.

For example, a 128GB iPhone 13 Pro is $41.62 a month. Add AppleCare+ for $8.29. The only difference between that and the Upgrade Program is that you are not guaranteed a 50% trade-in. The money you saved on your prepaid is probably worth much more than any loss there -- if there even is any; I expect iPhone 13 trade-ins to have excellent value.