The kiosk will not delete an upgrade until you actually change the flights. A warning will appear when you select change flights option from the main page that says something like "by changing flights you will lose your confirmed upgrade, do you want to continue." Clicking yes to that warning does not remove your confirmed upgrades. On the next screen, where you are presented with the available options, you have the choice to select a new flight or go back. When you go back your original flights with upgrades are still there.
I don't know the answer to your specific question regarding an earlier first segment but keeping the second segment. The kiosk may rebook the whole itin - meaning you lose the upgrade on the second segment. You may have better luck with an agent in that case.
I fear clicking to fast when I'm just playing around with options...