Hey, magenta4757730! First off, I'm glad to read you were able to switch over to T-Mobile ONE Military. Thank you!
Smartphone Equality should kick in on the month after the 12th month of consecutive on time payments -- are you just at one year, or have you gone over by a bit? It will qualify you for the lowest possible down payment -- so it's not always zero; but $480 is high for just the down payment on the S9+. IIRC, for well-qualified, that down payment should be $120 plus tax.
Is it possible that there's another amount added in there? Were you processing a JUMP! Claim (paying a deductible on your existing phone in addition to the down payment on the new phone), or do you have remaining balance on the S8 that needs to be settled to get you to 50% paid off? If neither of those situations are happening here; and you're in the 13th month after 12 consecutive on time payments, then *please* Contact Us so we can take a look at your account and get to the bottom of this! Especially while the Military Members get 50% off a Samsung Galaxy offer is still running!