This question already has an answer here:
Asking purely out of curiosity:
Is there any research or publication in Modern Physics that considers the possibility that the speed of light has changed since the Big Bang, and not remained constant (as what we know it today)?
By changed, I mean either gradually or abruptly, though I guess that the former makes more sense.
Now, I am guessing that the answer to this question might be more of "a matter of terminology".
We can say that the speed of light hasn't changed in the perspective of everyone who is within the given space, but since this space by itself has expanded, anyone theoretically "outside it" will "see" that light is in fact covering a larger distance per time unit (larger than what it used to when the given space was smaller)... Or we can say that it has slowed significantly, while the universe itself hasn't expanded at all.
On the other hand, I might be speaking nonsense here, so I'd be happy if one of you out there (or here) could enlighten me on this...