So I had a very similar situation right now.
I bought for October 10th, 2PM Day/Night a few weeks back. At the time I felt like I probably didn't need Express, and could just upgrade.
Now I'm really thinking back on it, and I want the express. Hell, I want the Unlimited Express which is an option this year.
There's a problem: it's only available as a bundle with tickets. And even worse, Unlimited Express is only available for the after 7PM ticket, and is not with After 2PM.
So I ask over email if they can upgrade, the answer is no, I need to repurchase and ticket and call for a refund. So I call up USH and they tell me that I can not upgrade over the phone, or online period. Either I stick with After 2PM, purchase a one-time 2PM and call for a refund (already what a migraine) or take my chances and see if an upgrade for Unlimited Express is available since I wouldn't be able to upgrade with the one-time Express after 2PM, apparently. Maybe I would, maybe I wouldn't.
This is outstandingly bad logic, especially for an event that's known to sell out its express ahead of time. I'm so frustrated right now that I'm just unsure of what to do.