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Using New Releases of DocBook
For each new version of DocBook, you can expect that the version numbers mentioned in all the DocBook files and their formal public identifiers will be updated. At a minimum, these changes will require you to change any public identifiers used in your documents and variant DTDs. (Note that dbgenent.mod does not mention a version number since it is expected to be edited by users.)
When DocBook is revised, you will need to assess whether you want to use any new features in the revised version and whether (for major revisions) any backwards-incompatible changes will require you to transform your existing documents to stay in conformance to the new version.
In addition, if you have created a variant DTD by means of a customization layer (see the Customizer's Guide), you will need to assess whether your changes are still valid. For example, you may have created an extension by adding a special synopsis element, which has now been added to the original DTD so that other companies can use it too. In this case, you would probably need to discard the customization to avoid parsing errors. For another example, DocBook may have added some list elements in the new version that you would prefer to remove from your variant, which would require a new customization.