Having been quite the aspiring canonist back in the day, I pulled out my copy of the Code of Canon Law for the Western Churches.
Can. 1364 -Ãƒâ€šÃ‚Âº1 An apostate from the faith, a heretic or a schismatic incurs a latae sententiae excommunication, without prejudice to the provision of Can. 194 -Ãƒâ€šÃ‚Âº1, n. 2; a cleric, moreover, may be punished with the penalties mentioned in Can. 1336 -Ãƒâ€šÃ‚Âº1, nn. 1, 2 and 3.
Now, like civil law, canon law has it's conservatives and liberals, and cross referencing this particular reference to a purple book I have on canon law, (I can't remember the name), this particiular canon extends to anyone who leaves the Church of Rome. Orthodox or Protestant. At the time of the writing in 1983, Orthodox were still considered 'schismatic'.
However, as i can remember, recent proclaimations exist, namely by John Paul II, that all excommunications of Orthodox have been rendered null, but I'm not sure if they exist on people who leave their existing faith under Rome, and become Orthodox.
PS Since Catholics dogmatize papal infallbility, and since not belieivng in a dogma constitutes heresy, one would be considered a heretic since he rejects the dogma which he is held under as a Catholic. This would satisfy the demands of the canon, as becoming Orthodox, you would of course reject that canon
Ok I pulled out my eastern code of canon law. In it it says :
Can. 1436 A person who denies some truth that must be believed by divine and Catholic faith, or who calls it into doubt, or totally rejects the Catholic faith, and does not reconsider, through legitmately warned, is to be punished as a heretic or an apostate with a major excommunication.
2. In addition to these cases, whoever obstinately rejects a teaching that the Roman Pontiff or the College of Bishops, exercising the authentic magisterium, have set forth to be held definitively, or who affirms what they have condemned as erroneous, and does not retract after having been legitimately warned is to be punished with excommunication.
Can. 1437 A person who refuses subjection to the supreme authority of the Church or communion with the Christian faithful subject to it, and does not obey is punished as a schismatic with a major excommunication.
It seems these are directed towards people who leave Catholicism for Orthodoxy.