The changes from version 2.08.xx to 2.09.03 are changes of the standard Hippo namespaces.

The new namespace revisions with their full names (of URL type) are:

hippostd	http://www.onehippo.org/jcr/hippostd/nt/2.0
hippogallery	http://www.onehippo.org/jcr/hippogallery/nt/2.0
frontend	http://www.onehippo.org/jcr/frontend/nt/2.0

In CND files, these namespace URIs must be updated. These CND files are normally located in your projects directly in the src/main/resources directories (unless you have manually changed this location in the pom.xml) or in the old location src/main/resources/org/hippoecm/repository.  Any file ending with .cnd should be updated. When forge plugins are used, they may need to be upgraded as well.
This also applies to content and/or configuration XML, when JCR namespaces are declared as XML namespaces. (note that these declarations are not necessary for valid XML, so removing them is also an option)