Variant changes
Customers can change their selected product variant through the Order Editing portal. Customers will be change to variants when there is stock available in the selected variant, the portal shows when a variant is available or not based on stock levels in Shopify inventory.
Only if you have chosen to ignore inventory levels will customers be able to change to a product variant that does not have stock available. You can learn more about this feature in the "enable quantity changes" doc.