Six Flags Season Pass Upgrades

If I go to a six flags (specifically six flags new england) on a discount admission (through work) can I still upgrade for $15 to a season pass, or do I need to have regular admission to do that?
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The legend lives!

You have to pay full price at SFStL, and I would assume it is that way at all the parks. The idea is that the one day admission price plus the upgrade charge equals the price of the season pass.