Pete I don't think this sort of propagation is mode-specific.
Why do I need to switch over? Not that I have such a switch, just asking.
Of course it's not mode specific. Propagation and MUF are never mode specific. However, you get a much better signal to noise ratio with SSB or CW when trying to work the "greyline" DX. With your AM rig, narrow the bandwidth, add a speech clipper, and you're good to go working long-haul. The DX station on the other end probably has no interest in trying to hear "great" sounding audio when working the "greyline".