The differences make sense as long as is functional to your environment.

If you dont upgrade the agents, all job will be marked yellow with the notification to upgrade agent to current release.

Read the differences in 12,5 and then decide

http://www.symantec.com/business/products/sysreq.jsp?pcid=pcat_storage&pvid=57_1

This is the best document I could find comparing the versions: http://eval.symantec.com/mktginfo/enterprise/other_resources/b-bews12.5_upgrade_matrix_081408.pdf

You would need to upgrade the agents and that would require a reboot of each server.

After reading the comparison chart, is there any specific reason you plan to upgrade? I run 12.5 because I run SQL 2008, for example. The reason I mention that is that a number of people, myself included, have experienced significantly increased times for our backups. Mine used to take about 7 hours, but now they take 9.