I think it would depend on how the observer is affected by the changed speed of light. Put yourself in the position of a omniscient being who is unaffected by the change. To you, the speed of light has changed and you could measure the difference in light travel time from point A to point B. But to the non-omniscient who are affected by the light speed change, their concept of distance would change with the speed of light. I think the change in distance would be proportional to the change in the speed of light: ie. no net change in measured frequency. Think of the Doppler Effect where only relative motion can be detected (to the first order).
I think a far more interesting concept is the idea of an anisotropic speed of light. Local differences could be measured.
The laws of the universe are uniform in all directions, or else!