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.
After April 2013 further security updates to JRE 6 will only be available through My Oracle Support and will require a support contract with Oracle. By default, Java applications that are accessed through a web-browser or webstart will use the latest version of Java on the system.
It is possible for an application to specifiy the Java version required.
In December 2012 Oracle will start to auto-update a sample of users from JRE 6 to JRE 7 to evaluate the auto-update mechanism, user experience and seamless migration.
Oracle will then start auto-updating all Windows 32-bit users from JRE 6 to JRE 7 with the update release of Java, Java SE 7 Update 15 (Java SE 7u15), due in February 2013.
Those users will not see additional updates as part of this upgrade, instead users that are on the latest version of JRE 6 (6u35 or above) will not be asked to auto-update until the next security update, due on February 19, 2013, rolls out.