With the Assign action, it does not pull in the code so what you have to do is "hide" the creation of the code action in another process. Typically what I do, I publish a button like Submit or something similar and put the action in there. Or, if the next step in the process as a Process, I put the action in there. Lastly, create a view which is filtered only on members that don't have a code, that way code creation action will be fast and will only run on members that need updating.
Since the Assign action does not update the code, you have to put the action that updates the members in a different process. For example, after the user does the assign action, have the user click a button "Submit" or "Finalize". This button is actually a process and the first action within the process is to update the records (List members) that don't have a code.