A Latin Rite Catholic who attends an Eastern Rite church, even if he enrolls in the parish, goes every Sunday, etc., is still bound by the laws of his own Latin diocese/rite (days of abstinence, rules of fasting, Holy Days, etc.), and is not bound by the Eastern Rite rules (e.g., The Great Fast, etc). He can voluntarily choose to follow them, but he must follow the laws in force in his own diocese. If he goes through the process of changing rites, which entails getting released by his own bishop and accepted by the Eastern Rite one, then he would be bound by the Eastern Rite rules.