Great question. I don’t think it is possible to copy from (Single Line Text) ---> to (Drop-Down Select) because HubSpot doesn’t know what to do if the Single Line Text option isn’t one of the Drop-Down options.
I also noticed that when I copy from (Drop-Down Select) ---> to (Single Line Text) for some reason it copies the Internal Value rather than the label. Ex: When I copy Company Industry (Drop-Down Select) --> to Contact Industry (Single Line Text), it will list the Contact Industry as “LAW_PRACTICE” instead of “Law Practice”