I don't know if this was posted before. Some spanish judges wrote an article on the topic, explaining what they accept and what not, with examples.
Gu?a de Alteraciones
(Click here to see what Google Translate has to say on the topic)
I don't know if this was translated somewhere. To put it short, the authors only accept alterations keeping names, mana costs, borders and most of the text, and leaving the original art recognizable. They insisted on the card border's issue several times.
Though we all know this is up to the head judge and stuff, it seems that judges will be pretty strict about alterations at GP Madrid, considering it's competitive event. Through the authors just say something like: "I'd allow this at a shop tournament, but not at GP".
Ссылка на источник: mtgthesource