Yello and Yellow Fang,
I think you are both slightly missing the point. How can a film which is firmly set in history "reflect the fact that times have changed". And you can't say that people of that era were racist. By our standards now, yes but by their's, no. Remember that up until the seventies one shade of brown was officially know as "nigger brown" and nobody tied it up with a group of people.
Sensibilities about the word have essentially been imported from the US where it has always had the most terrible, unsavoury connations. There is no going back on that in UK culture now, nor should there be IMO. But for what it's worth, I cannot imagine any sensible black person being offended by its use in such a film. Probably the most which would need to be done is for a note to appear as part of the title sequence, explaining why the dog's name ended up having a role in the actual raid and that that was possible because in those days nobody would have considered its use as racist.