Why is this?
FLIR IR Monitor tries to detect changes of lenses in order to make the camera use proper calibration data. However, sometimes this detection fails. Changing the temperature range after having changed a lens can thus make the image quality and temperature accuracy degrade. If this happens, you need to disconnect the camera from FLIR IR Monitor and reconnect it again.
Workaround:
To avoid this, disconnect FLIR SC6x0 and FLIR SC6x5 cameras within FLIR IR Monitor while changing the lens. (You do not need to unplug the cable or close the program.)