XF 2.1 Copy Custom User Field Data after Import

Movie Prop Sites

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We are merging two forums (both Xenforo). Sadly, we created custom user fields for both with SLIGHTLY different names, so now instead of one custom user field for "First Name" we have two, one with the field ID firstname and one with the fieldID first_name. I want to copy any datat from firstname to first_name. Can someone help me with the SQL script that would copy the user data from one Custom user field into another?