This is because the Publisher replicates its database to all subscribers. Configuration changes are written to the Publisher and then replicated to the other nodes. In order for this replication to take place the Publisher database needs to be upgraded first.
Take a look at the info on database replication at the following link.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/10_0_1/ccmsys/CUCM_BK_SE5FCFB6_00_cucm-system-guide-100/CUCM_BK_SE5FCFB6_00_cucm-system-guide-100_chapter_0110.html
Brandon
