WD TV Live latest model with Wifi and Lan, how can i use both

Yes, if you have a wired connection then change to a wireless connection and the wireless drops it will go back to the wired connection.  At least as tested.  But when the wireless signal returns it will not automatically reconnect to it.

But this is not instantaneous, and can take a few minutes to notice the drop and reconnect to the wired network, this will also cause it to rescan the media library.