Hi Mike,
In my tests on 9.6 SP1, if you open your AV settings, enable Full scan in scheduled tasks, and then save/exit the av settings, when Vulscan is ran on a client that is assigned that behavior, it will download the updated behavior.xml. So yes, your agents should pick up this change made to the Agent Setting provided they had this setting assigned to them when they were installed.
If I have to push it, how would I go about finding all the agents that need to be updated.
Assuming your agents are utilizing the same configuration/installs, you could run a query based off of the the inventory value LANDESK Management | Agent Configuration Name. Set your value to '=' the agent configuration that is using the AV settings you need changed. (Example: Query where LANDESK Management | Agent Configuration Name = MyAgentsWithAV). The results will be machines who have that Agent, and you can add them to the scheduled agent settings change.