What about developers who work on maintaining existing Java EE applications? Can new versions of application platforms also serve this development community?
Yes, this is possible and JBoss EAP is well suited to continue supporting existing Java EE applications. Java EE 7 is the version supported by JBoss EAP 7. While it is possible for some Java EE 6 applications to be moved to a Java EE 7 application platform, there can be compatibility issues that prevent this. In those cases, developers would continue to use JBoss EAP 6, which is a Java EE 6 certified application platform. In mixed environments, JBoss EAP 7 domains can manage JBoss EAP 6 domains, providing administrators with a management tool for both.
Pivotal does not offer a Java EE certified application platform and as such can be quickly dismissed as an option for developers maintaining Java EE applications. This is a common complaint Red Hat hears from customers who have considered the Pivotal platform.
IBM by comparison recommends support and maintenance of existing Java EE applications using the traditional WebSphere Application Server. While some existing Java EE 6 and Java EE 7 applications might be able to run on WebSphere Liberty Profile, there may be legacy compatibility issues that prevent the move.
In addition, while IBM Liberty Profile can concurrently run Java EE 6 and Java 7 features, not all are compatible with each other. There are multiple incompatibilities that may force you to run IBM WebSphere Liberty profile exclusively in Java EE 6 or Java EE 7 mode.
Oracle WebLogic Server is similar to JBoss EAP insofar that each product version is only certified for a single version of the Java EE specification. Both JBoss EAP 7 and Oracle WebLogic Server 12.2.1 are Java EE 7 full platform certified. Earlier versions of products from both companies offer Java EE 6 full platform certification.