Do I need to reformat the hard drive when updating a Mac? Do I need to reformat the hard drive when upgrading from OS X Mavericks to OS X El Capitan?
You do not need to reformat your drive unless you want to do a fresh install.  You can upgrade in place without having to format.  If you find you are having problems after the upgrade, you can then reformat and start from scratch.

However, if you do reformat, ensure all of your documents and programs are backed up so you can restore them as it will wipe all data.