You are NOT automatically excommunicated according to the Society. You disassociate yourself if after having taken the transfusion, you are not repentant. Your actions are said to be the cause of the reason that you would not still want to be a JW. If, after the action, you felt terrible and a commitee believed that you're sorry for taking blood, you're not dissed.
They renounced the Blood Transfusion issue??!!
You see, their logic is basically this:
They did not shoot you in the head. What happened is you stood in front of the gun while they pulled the trigger knowing full well that you were in front of the gun.
Therefore they are not responsible for your death, and you were the one who caused it.
Its all about semantics. The same action results in the same outcome. Nothing has changed, just the way the spin doctors present it.