updating one column data into another column in same form

updating one column data into another column in same form

Hi,

    In my form one field is string type and another field is lookup that means longList.So i want to update all data from String field to long-list based on validations.I have created one custom action for that but i am getting some errors.I think it is problem of type,i am updating string to Long-list.Is there any solution for it.

Thanks.