After a cart2cart transfer into magento 2 we noticed that all the attributes were set to text. I found a quick way of switching the attributes back to dropdown in phpmyadmin worked for us.
open table eav_attribute
Click edit on the attribute you need to change and change the frontend_input from textara to select.
Here is a list of the attributes codes.
textarea – text area
select – dropdown
date = datetime
text feild – text