OS X users can only have one JRE available through web browsers so OS X users can not have more than one JRE available through the web browser and will only only have JRE 8 available through the web browser after the upgrade.
Unless you have a support contract with Oracle or a license for Java SE Advanced products you will not be able to get security updates for JRE 7 from Oracle after April 2015 and it is not advisable to keep older versions of Java on your system.
Oracle strives to make new versions of Java backward compatible with earlier versions and the vast majority of Java applications written for older versions of Java will work –without any change- on newer versions of the JRE. Any change in the code –even if only to fix a security vulnerability- has the potential to cause an application to stop working properly.
To revert back from JRE 8 to JRE 7, first uninstall JRE 8. Until Jul 2015, the latest version of JRE 7 can be found on the Download tab of the Java SE section on OTN.
During the automatic update from JRE 7 to JRE 8, if a user only has one version of Java 7 the auto-update process will replace that JRE 7 with the latest version of JRE 8 leaving only JRE 8 in the system.
OS X users can only have one JRE available through web browsers so OS X users will only have JRE 8 available through the web browser after the upgrade On Windows, the auto-update process updates only the latest JRE on your system.
Versions other than the latest will not be removed as there are cases in which a user, particularly enterprise users, would need more than one version of Java on their systems.
If you would like to remove older JRE versions from your system please use our Java Uninstall Tool to identify and remove older versions.
The Java auto-update mechanism is designed to keep Java users up-to-date with the latest security fixes.To achieve this goal Windows and OS X users that rely on Java’s auto-update mechanism will have their JRE 7 replaced with JRE 8.The Java auto-update process updates the latest version of Java on a user’s Windows and OS X machine.Nevertheless on Windows systems, if you would like to continue to have both versions, JRE 7 and JRE 8 can be installed on the same computer.
OS X users can also have multiple JRE versions available for desktop applications but only one of those versions can be used through the web browser.
After Jul 2015 further security updates to JRE 7 will only be available through My Oracle Support and will require a support contract with Oracle. For Windows users, by default, Java applications that are accessed through a web-browser or webstart will use the latest version of Java on the system.