Under your TV "Device > Settings > Adjust inputs", make sure your TV device is configured to use Method 2 (discrete method - default method for the LG42LK450), and review your activities to make sure HDMI1 is selected for the TV input.
Your post is confusing. Is your Harmony software setup configured to use the new LG42LK450 or they old TV?
Note: When you replace a device in your configuration, don't forget to remove the old one.