I don’t have any personal experience but from everything I’ve read upgrading isn’t any issue. You’ll just need to go to Guest Services. I am pretty sure it is recommended to use the tickets first (ie, enter the parks with them first to activate them to the current value for that type of ticket) so you’ll get the most value for them. I saw you asked this in the US forums and I am sure someone will answer and have more definite advice. The mods and other regulars on there are awesome and very knowledgeable.
When are you going? I’ll be there in just under 2 weeks and have to go to guest services as I need to upgrade my tickets from Undercover Tourist to Annual Passes. I can ask about the AM tickets then too if you’d like.
Not sure what you mean by UA but we upgraded the Universal Studios Orlando 2 day 2 park tickets and 2 day 3 park tickets to annual passes in October no problem. We just went to customer service inside the park. Upgrade costs were $10 for the 2 park and nothing for the 3 park.
I did it in Universal Studios. Go to guest services before you leave the park to upgrade the tickets. It is on the far left side of the exit on your way out. I believe IOA closes earlier than US in August, so I upgraded in US.
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