Should I upgrade?
While the DITA CMS 4.3 ships with DITA 1.3 integrated, you can use DITA 1.3 with earlier versions of the DITA CMS as well. These instructions apply to versions of the DITA CMS that do not ship with DITA 1.3 natively integrated.
If you want to take advantage of the new DITA 1.3 features, which include scoped keys, ditavalref, the troubleshooting topic type, Release Management (not to be confused with the DRM module of the CMS), and the XML mention domain--among others--then yes, you should upgrade. If you do not need any of these features, you do not need to upgrade right now.
If you upgrade to DITA 1.3, all content that you created using DITA 1.2 remains valid. You do not have to make any changes to it. Also, even if you upgrade, you are not obligated to use any of the new DITA 1.3 features. However, unless you have specific use cases for upgrading to DITA 1.3, IXIASOFT does not recommend doing so.
While it is theoretically possible to use two versions of DITA at the same time in the DITA CMS, IXIASOFT does not recommend it and is not prepared to support it.
Dynamic Release Management (DRM) concerns for upgrading to DITA 1.3
All of the DRM functionality is defined within the com.ixiasoft.dita13.dtd plugin. By installing this plugin in your system configuration (as instructed in Add the DITA 1.3 DTDs to the DITA CMS), you automatically take care of any DRM concerns with respect to DITA 1.3. None of the pertinent files should be a part of your custom DTD plugin; therefore, you should not have any DRM-specific changes to make in your custom DTD plugin.