If you’re considering exchanging a pre-ordered iPhone 6 for a different model, just know that your mobile carrier upgrade qualifications may be delayed during the exchange. To get the subsidized price of the iPhone 6, you need to re-up your two-year contract with your carrier. But during a return/exchange of a pre-ordered iPhone 6, technically you already used your carrier upgrade. So you need to wait until your carrier processes your return and reinstates your upgrade eligibility. Only then can you purchase another iPhone 6 at the subsidized price.
You’ll have to do all of this when the iPhone 6 releases in-store so it’ll already be chaotic with inventory varying greatly. Do you really want to do that or can you live with your pre-ordered model?