If it is a configuration change, then you can push an agent update. For service packs, you can either rebuild the setups and push the agent again (my preferred method) or you can use security and patch manager to push the update.
Yes, since you updated your core to v9, you do need to push out new agents. Alway click on the "rebuild all" icon in the console in the agent tab before doing so.
If you were going from LD 9 to LD 9 SP1 you could use the patching tool to push the SP1 patch to the client afterwards.