I recommend reading this AEM topic that talks about upgrading AEM:
https://dev.day.com/docs/en/cq/current/deploying/upgrading.html
https://dev.day.com/docs/en/cq/current/deploying/upgrading/planning.html
If you want to take advantage of some of the latest features in AEM 5.6 - you may have to update some of your OSGi bundles with AEM 5.6 JAR files. For example - assume that in 5.5 -- you wrote a custom workflow step using Java APIs located in this Java package:
com.day.cq.workflow.exec.*
In AEM 5.6 -- you can build a custom workflow step using Java APIs located in this package:
com.adobe.granite.workflow.exec.*
In this situation - you have to update your OSGi bundle (that represents the custom workflow step) and reference the newer AEM JARS in your bundle.
Also - take a look at this forum thread - its related:
http://help-forums.adobe.com/content/adobeforums/en/experience-manager-forum/adobe-experience-manage...
Hope this helps