Let’s think logically about this.
They’ve already released some merch for this year that hasn’t revealed anything about what’s coming to this year’s event. That means, merch-wise, they can sell stuff in the parks without announcing anything. They would just stock new merch the night after stuff is announced.
Now, the Tribute typically has sections themed to specific things that are featured at that year’s event. One, it doesn’t have to have that. It could be somewhat generic sections more like the Mardi Gras store. If it does feature IPs, it won’t likely feature ALL the IPs. That means Universal would only have to announce the specific IPs featured in the store.