Damo is right that Universal does not discount an initial annual pass. But, Universal's current upgrade policy will allow you to save some money if you buy a ticket in advance and then upgrade it to an annual pass during your trip and Guest Services. I upgraded an old 4 day park to park ticket I purchased from Orbitz to a Power Pass for no extra money. I've not looked at all the tickets out there, but the 2 day + 1 free that DOES NOT expire from Undercover Tourist is a good ticket to upgrade. Undercover Tourist has been giving 3 day tickets for this. The 2 day + 1 free tickets that do expire are 2 day + 1 free tickets and don't seem to be work as much when upgrading. Universal seems to be charging the difference in gate prices between what you have and what you want when upgrading.
I don't have any first hand knowledge with those tickets and haven't read any reports on people upgrading them here. Based on the current 2 day + 1 free promotion being treated as a 2 day ticket for upgrade purposes I would guess you'd pay the difference between the gate 1 day price and the annual pass price. But, that is strictly a guess. Some deep discount tickets aren't upgradable at all for example USSSA tickets. So, I'd double check with Universal to see if those tickets were upgradable.
The Undercover Tourist promotion for 2 days+1 free that does not expire has been fulfilled with 3 day tickets since it's an Undercover Tourist promotion not a Universal Promotion. That's why it's been being treated like a 3 day ticket.