If I'm understanding correctly you want to change the name of the FIELDS not the values in the fields.
I think RENAME FIELD is what you need to use in your Load Script. It would allow you to read in the Old Field Name and the New Field Names and then apply that. Example 2 in the following link will show you that and don't mind the fact that the example loads from a SQL table, you can use your XLSX file to create the FieldMap table.
