custom fields won't modify

If you create a custom field (day single dropdown) and save it and apply it to a product, and then go back and change your mind and modify it to be text, you can change the default value, however in the product it will not change.
Even if you delete it from the product category and re-enter in the old default value continues to show up even though it all looks right in the custom field page.
Only way out is to completely delete custom field and start over again.

Hi,

The default value of a custom field applies only for when it has never been selected. Once you select it and save the item, the item has that value, be it the default value of the custom field or not.
Suppose that you have the default value of the country address field to France. Someone registers and use that country for his address because he lives in France. Then, you decide to change the default value to USA because most of your customers are from the USA. But of course, you don't want the country of the users who are located in France to change to the USA because you changed the default value of the custom field.
Well, it works the same for all the custom fields.
Of course, for new items, the current value of the custom field applies.

That makes perfect sense, maybe the answer is to not make it appear that it can be modified.