Scenario
Your product has multiple variants, and preorder is only enabled for some of them.
For example, a t-shirt has these variants: S, M, L, XL, XXL — but only L and XL are set for preorder. When you view the product page and select L or XL, the Add to Cart button doesn’t change to Pre-order as expected.
Solution
Enable preorder on the first (default) variant, even if you don’t want it to display as a preorder.
To prevent the default variant from being shown as a preorder, disable overselling for that variant. This ensures the PreOrder Now app script loads fully without affecting how that variant appears on the storefront.
Why This Happens
When the product page loads, the app checks if the default variant has preorder enabled. If it doesn't, the app doesn't fully load the scripts required to support variant changes for preorder.
Some Shopify themes use AJAX to switch variants without reloading the entire page. Since our app loads its full script only once (on page load), variant switching via AJAX won’t trigger our script again—unless the initial variant is set for preorder.
Enabling preorder (with overselling off) on the first variant triggers full script loading, allowing the app to listen for variant changes properly and display the Pre-order button as expected.