To associate only new monitors, follow the below steps:
[ Note: Do not click on Associate & Overwrite - this will associate the new monitors and as well as overwrite the existing monitor configurations in the device ]
Associate option adds the new monitors (if any) to the device without affecting the existing device level monitors/thresholds.
Apply & Overwrite option associates the template to the device and overwrites the exisiting device configurations/monitors with those in the device template