The promotional period is only for a limited time. If you look to upgrade to the PH after the purchase window is over I'd expect the system to ask for the full price difference from what you'll pay for the dvc 7 day non-ph pass and whatever the current going price of a PH will be (hence, no savings.)
Now, does this mean that if you use it once to enter a park and then upgrade to the PH that it will still ask for the full difference? I can't answer for sure, but indicators about anything being bought before February 12, 2017 being price locked to that price/time tells me Disney closed that loophole in the system.