Set Value

as far as I remember, changing the dropdown values set for a dropdown will not lead to changing the values that were set for the dropdown field in the records in the database
also, please specify what "first value was chosen" means to understand  the copying start condition
e.g. any time when dropdown value is selected for the dropdown field of the record do copy the previous value to the dedicated text field, or perform copying only when the value for the dropdown field is set for the record for the very first time? 

If you want to track the original value only you can do a simple logic hook that copies the dropdown value to a text field only if the text field is empty.

That way you copy the first time that the field is set, and then it stays the same from there on regardless of what happens to the dropdown field.

Francesca