Diablo 1 Lazarus' Lair

[[vL]Kp]

Finally, a question that people won't like, but I'd be interested
in this one: Is it considered a big cheat if I change the skin
of an item I acquired legitly? (I want to change the armor type
of the Naj's Light plate [if it ever drops] to Mail armor, because
I like the Mail skin on Sorceror much more than the Plate one.
It's basically just a graphics modification).

As best I can recall, the appearance of your armor has no effect on your character's performance (block rate, hit recovery rate, etc.), and if you do it right, it should have no effect on the experience of other players in the game. One of the common tests for "cheating vs. not cheating" is whether the proposed change has any effect on gameplay. If the game plays in exactly the same way with and without the change, then I see no problem with this.

I suggest making the change by modifying the graphic string selector to always select medium armor instead of looking at what the player is actually wearing. Some additional work may be needed to avoid having it modify the appearance of your companions.