Assume the original Excel have such columns.
A1, A2: not pivoted
B1, B2: pivoted
The result depends on how the Excel file is changes.
 If column names remain the same in the Excel file, and only data updated, there will be no issue.
 If a new column is added into the Excel file, it will become a not pivoted field in Tableau workbook.
 If B2 is renamed to B3 in the Excel file, B3 will become a not-pivoted-field in Tableau workbook.