I agree that it's best to communicate the whys on if there is a significant difference in design for it to be more accepted. I haven't paid much attention to the Arbiter's new life either, but I'm sure they communicated it in the comics or the new books what he's up to now. If not, then they should. I'm sure someone here can speak on if he has a different role. If so, that would explain his armor change. I find the change exciting, because it makes me say, "oooh, what's Arbiter doing now? He appears to have some new leadership role or something."

In regards to the Chief, personally I don't consider his armor change significant, even if it's mechanically very different. Arbiter can't have the "artistic liberty" explanation because his armor is VEEEEERY different, in both color and design. Gold versus silver isn't artistic liberty IMO. I personally feel like Chief's is less so, and was just made to be more realistic. (I feel like I read how if his armor was designed like Halo 1, moving around would kind of be an issue). For me, that didn't bother me.

I think 343 has learned, or are surely learning, to explain things before significant artistic liberties occur. The new Jackals in Halo 4 are actually new Jackals now, versus artistic liberties. Canon wise.