Are cards required to be sleeved in tournament settings? All this talk on blogatog has me wondering if changing the card back would have any judging ramifications.
Players are responsible for ensuring that their cards and/or card sleeves are not marked during the tournament. A card or sleeve is considered marked if it bears something that makes it possible to identify the card without seeing its face, including(but not limited to) scratches, discoloration, and bends.
This is the first part of what the Magic Tournament Rules have to say about marked cards. Sleeves aren’t strictly necessary, but there’s a fairly high bar to playing without them.
Personally, I’ve only ever seen players playing with unsleeved decks in limited events. I have never seen anyone play Standard or other constructed formats unsleeved.
As for tournament ramifications of changing the card back, it doesn’t just affect unsleeved play, it would affect all formats in which players would be playing with new and old cards together. Not all sleeves are fully opaque, and doing something like changing the Magic logo from blue to orange on the back of a card would be visible through the backs of some lighter colored sleeves, and that wouldn’t be a good situation. Players playing in these mixed formats would need to use fully opaque sleeves, and that’s hard enough to enforce when there are double-faced cards in the format. Having entire blocks with different backs would be a disaster and a huge headache for judges.