Long story short, I was going to Edge Up my M8 to a 6 Plus but backed out before the actual upgrade started because of a spending limit. Paid the Edge Up amount at the very beginning of the transaction because we couldn't even get started without it. So I ended up walking out of the store minus 99 bucks but under the impression I could upgrade whenever I wanted seeing as I paid the Edge Up portion (not the full balance, but the amount to upgrade early). Now, here I am two billing cycles later with a fat $300 equipment charge for not returning the device that never got upgraded! Called a few times and none of the reps could answer why I have to pay for a device that I'm still using and paying on. So caution to everyone, please make sure you are actually going to upgrade before you pay that Edge Up portion!
OOh darn. Look on the bright side. You can now sell the paid off phone on ebay and start clean with a new phone and Edge contract.