1. A meter is defined as: "the length of the path travelled by light in vacuum in 1 299,792,458 of a second."
2. The second is defined as: "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."
If we want to retain any frame of reference we are forced to say that in Setterfield's universe the meter is defined as it is today, by today's lightspeed. In the same way we have to retain the present day definition of time.
No, that's exactly what we can't do. The measured quantities can change, but how can a
unit change? What we could say is that if we use the speed of light as our standard for a meter, then a meter as measured by this technique is getting shorter, thus throwing off our efforts to measure distances; but if we let the units vary too (which is nonsensical) then we would have to say that a meter as measured by that standard always stayed the same length, namely a meter ... and we would also, now I come to think of it, have to say that the speed of light was constant, since it is defined in meters per second, no matter how much it actually changed.