Chenging a field type in admin

[tens1234]

Hello everyone, i am using prestashop 1.6 and overall everything is running smoothly. 

 

I am trying to prevent more human error when listing new products.

 

I want to change a field type in the backend, more specifically the "Displayed text when in-stock" and "Displayed text when backordering is allowed" are text fields. This has resulted in a lot of different similar but different entries, this conflicts with various price comparison websites and has been a problem. 

 

I simply want to change this field to a dropdown with predetermined values. The website has two languages(english and greek) if that matters.