The short answer to your questions is that you do not need a separate backup before running AXPOR01.
First, let's look at the installation/upgrade process. You are installing version 14.0 as a new installation, which means you are creating all new database, operational and control files. There is no migration of the version 12.0 databases. So when the installation is complete, the version 12.0 environment is not affected and therefore does not need special backups.
Now, for the migration job - AXPOR01. This job will copy Datadictionary database definitions from the version 12.0 system and then apply them to the version 14.0 Datadictionary. It will also copy some statistical information from your version 12.0 CXX to the version 14.0 CXX. Neither of these alter the existing version 12.0 environment so nothing needs to be backed up here. The final piece of the migration activity is to turn off access to your application database in the version 12.0 CXX, and then connect the physical files used for the application database to the version 14.0 CXX. The data itself in the files is not changed, and only certain control blocks are changed to note that the new version 14.0 MUF now "owns" the files.
As a result, since no application data is being changed, there is no need to run a DFDSS backup, and there really is no benefit beyond what your normal backups provide. In the event you need to fallback from version 14.0 to 12.0, you will run the AXFFB01 job, and it will reconnect the files to the version 12.0 MUF and allow access to the database again. Re-implementing version 14.0 is then as easy as rerunning the AXPOR01 job once more.