Replace older version of Excel with newer version

The act of updating/upgrading Microsoft applications to current versions only changes the applications and their supporting libraries. Your existing data files are not affected. Always a good idea to have a current backup of your Mac so that you do have retrievable documents at any time.

You will not "lose" your older worksheets. Newer versions of Excel can open older versions of worksheets.

What you should be aware of, however, is that if you have truly old Excel worksheets in the .xls format, when you open them in a newer version of Excel that uses the .xlsx format you will get a warning message. You will then have the option of either continuing to save each such worksheet in the old .xls format or saving in the newer .xlsx format.

You should not lose them.