Hmmm, I checked my upgrage eligibility with AT&T on apple.com, and as I expected, if I buy an iPhone 6 Plus on 9/12, I will have to pay full price, and I can purchase it on 9/21 for $200 less. However, there is an asterisk above the full price "Buy Today" column as well as over the "Buy Sept 21, 2014" column showing that a new 2 year contract is required.
"**Requires new two-year AT&T wireless service contract, sold separately to qualified customers; credit check required; must be 18 or older. In CA and RI, sales tax is collected on the unbundled price of iPhone."
If I'm reading this correctly, I'll have to renew my contract for 2 years if I pay full price on 9/12. I hope this is an error or that I'm misinterpreting.