From past experience (I have only gone up to 6), I have found that if you use an educational serial on an upgrade, the upgrade will be 'educational' (which just means there is a educational splash screen.
I spoke to an Adobe customer service employee about a year ago and asked what one does to upgrade from an educational to retail version of Photoshop. She said you can simply call Adobe and tell them you are no longer affiliated with an educational institution and you would like to change to a retail serial number. They would provide one at no cost. This may have been misinformation from an uninformed customer service rep (I have encountered a few in my life)... but it may be worth calling Adobe to confirm.
I never tried this since I still have an educational affiliation.