The reference to customizations meant that if the JRE 8 installation was modified in any way by the customer, those modifications would have to be applied again on the JRE 11 installation. Such modifications are not very common, the most common customization would be adding a custom endpoint certificate in the JRE SSL truststore cacerts.
No customer action is required for regular SnapPacks. They are downloaded on-demand as required.
If there is a custom snap which is having issues under JRE 11, it might have to be updated and rebuilt. There are some web services packaging related changes in Java 11. To fix that, add the below dependency in the pom.xml and rebuild the custom snap
<dependency>
<groupId>com.sun.xml.ws</groupId>
<artifactId>jaxws-rt</artifactId>
<exclusions>
<exclusion>
<groupId>com.sun.xml.bind</groupId>
<artifactId>jaxb-core</artifactId>
</exclusion>
<exclusion>
<groupId>com.sun.xml.bind</groupId>
<artifactId>jaxb-impl</artifactId>
</exclusion>
</exclusions>
</dependency>
