Magento, by default, does not facilitate to change the attribute type.
The admin may require to migrate the attribute from a text value to a drop-down value without losing the values already entered for the products.
A custom script is required to convert attribute type from text to dropdown for Magento to overcome the default Magento limitation.
Check https://meetanshi.com/blog/convert-attribute-type-from-text-to-dropdown-in-magento/ for the stepwise method to convert attribute type from text to dropdown in Magento.