You do not have to upgrade all of your agents to version 9.x at the same time. When you upgrade a management server, you also upgrade the agent on the management server computer. Otherwise, you can upgrade agents on an as-needed basis.
If you installed a new version 9.x QDB and want to maintain existing agents, you must migrate the agents to the new QDB. For more information about migrating an existing agent to a new QDB, see Section 3.8, Migrating an Agent to a New QDB.