If you're using an antivirus package not supplied by Microsoft, but one from a different provider (popular examples: Norton, Kaspersky, McAfee, Panda, AVG), you might wonder if Microsoft might try to replace your preferred security product with its own (Windows Defender) during the Windows 10 upgrade.
Microsoft says it won't, unless your security subscription has expired. And, even then, the upgrade will give you temporary protection using Windows Defender until you can fix your subscription.
For Anti-virus and Anti-malware applications, during upgrade Windows will check to see if your Anti-virus or Anti-malware subscription is current. Windows will uninstall your application while preserving your settings. After upgrade is complete, Windows will install the latest version available with the settings that were set prior to upgrade. If your subscription is not current, upgrade will enable Windows defender.