Whenever you change a field type, the system will prompt you to let you know that there could be potential data loss. This means that you will lose the values in your fields if data in the field doesn't meet the criteria of the field type. For example, imagine that I have a field (type text) called "Name" and it had the value "Alan" in it. Now if I change this field type to "number" the values in my field will be lost because the value "Alan" isn't a number and you can't enter letters into a number field. In your example, you are going from a list to a text field type so the data in there should remain since a text field accepts both letters and numbers. 

Of course there is always risk involved when making changes to metadata fields like that since it can affect multiple files. I would recommend to back up your files before you make the change if possible. Another thing you can try is to add a new field to the template. Have a workflow put the value of that list field into your new field as a placeholder. Then make the change to the field type. If for some reason data gets lost, you still have the original value in that new field that you created.