When using an Ecall, I almost always shut it off eventually when I have a coyote coming in. I Basically shut off the caller once I beleive the coyote to be with-in hearing range of my lip squeek.
There are no hard set rules. I've seen coyotes sit around and listen to several different sounds on the same stand, and then all of a sudden, a sound that interests them gets them to come. I've gone from using a hand call on stand, to the Ecall, and visa-versa, and had coyotes come.
I guess what I'm saying is, at 800 yards, I would've stayed with the same sound, but I too would've changed sounds once I thought they could hear my lip squeek. And really, it's the same scenario as you discribe. A drastic change of sounds, but rarely do I have them stop once I go to lip squeeks, although, it has happened before.