Protect Data Filled Cells on Save Only
 

Is there a way to protect data filled cells when the user saves the
spreadsheet? I would like the user to have the opportunity to open the
spreadsheet, enter data in the empty cells, have the oportunity to go back to
the any of the data that was just added upon opening and change if needed,
and then have all cells with data in them locked when the user saves the
spreadsheet. Of course, when the user opens the spreadsheet again, I would
like all data filled cells to remain locked.