AT&T's made it pretty clear how this upgrade sitch is going to go down for existing customers moving to an iPhone 4 on the 24th of the month, but if you want to double-check and make sure you're good to go (we can't blame you), there's an easy way that was briefly mentioned yesterday: dial *639# on your current phone. You'll be returned a text message from AT&T detailing your situation -- either you'll be told you can get a full upgrade price (meaning $199 / $299 for the 16GB and 32GB, respectively) or you'll be eligible at some particular date in the future. Not all's lost in the latter case -- you can still pay $200 more for the phone as an "early upgrade," which is less than the no-commitment price of $599 / $699. We'll have a more comprehensive breakdown in our full iPhone 4 guide coming later today, so stay tuned!