You are not required to modify existing applications that do not use features available in the new Oracle Database release.
Existing applications running in a new release of Oracle Database function the same as they did in earlier releases and achieve the same, or enhanced, performance.
Many new features and enhancements are available after upgrading to the new Oracle Database release. Some of these features provide added features and functions, while others provide improved performance. Before you upgrade your applications, you should review these new features to decide which ones you want to use.
Oracle Database New Features Guide for information about the new features available in Oracle Database 12c
http://www.oracle.com/technetwork/indexes/documentation/Oracle Technical network (OTN) for information about using individual Oracle Database 12c database products or options