Hey there Richard,
Welcome to planDisney and thanks so much for this question. I certainly understand that plans change - luckily, it's a pretty straightforward process to get the dates of your visit changed.
As you noticed, 1-day tickets are on a tier system (tier 0-6) based on the date. Higher tiers are a bit more expensive but have more days available to visit, and from what I've seen, it looks like the higher-tier days tend to have a bit higher crowd levels as well.
There's a really helpful breakdown of the blockout dates for each 1-day ticket tier on the
Help with Tickets FAQ page, but it looks like you'd need at least a Tier 2 ticket to visit on April 15, 2024. Your next step will be to
call a Cast Member at the Tickets line who can help you upgrade your ticket and take the payment for the price difference.
From there, I like to use the
My Plans hub to confirm that my theme park reservation is booked for the date of my visit. This is also a great page to make or modify any other reservations you might need for your trip as well!
Thanks again for your question, Richard! I hope you have a fantastic time visiting the Happiest Place on Earth.
Have a Magical Day,
Greg