I suppose an export of the user file will provide all the data that’s needed, and it can be cleaned up to make ready for import using something like this:
  
  
    Jay has delivered the goods: 
It requires some competence with Linux / Bash in order to use it, and I’d strongly recommend doing a backup first and doing it at a quiet time as you can royally stuff up your user data if not careful. I trialled it on a staging server before using it on our production site. 
You might find this Data Explorer query useful in order to get the existing data for the user_field that you are targeting: 
--[params]
-- string :ucf_name = user_field_1
SELECT
    user_id,…
   
 
             
            
               
               
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