There are several ways NU lists of becoming the active translator for the series, which set an unbiased ruling on who the active translator is which is a good thing, but I think that in no case should a translator be using the previous translator's work without permission. So my suggestion is that if any translator copies over the previous translator's work or rehosts it in any way without permission, they would not be allowed to be the active translator.
So... You're saying that if someone copies 10 chapters of a site that went down (as in, they got those from Google Archive), and then translated 1000 chapters for a series, they shouldn't be listed on NU? Yeah, way to discredit someone's hard work, I don't see this happening~