On pg 145 of the DP 8 Installation and Licensing Guide:
After the upgrade, the Cell Manager, and Installation Server must have the same Data Protector
version installed. Although older Data Protector Disk Agent and Media Agent versions are
supported in the same cell, it is highly recommended that the clients also have the same version
of Data Protector components installed
So, as with previous versions of DP, you do NOT have to immediately upgrade your clients, but, you will probably get soem strange warnings about Unsupported Options until the clients are upgraded