Since the WTS prohibits the use of blood transfusions, why does it permit the infusion of albumin, clotting factors, and gamma globulins, all of which are derived from human blood? Since Acts 15:29 clearly refers to the old Jewish law of not EATING blood (Gen 9:4, Lev 3:17, Deut 12:16), and since the WTS has changed its teachings so many times on major issues like organ transplantation, the definition of "generation", the year of Armageddon, etc, etc, and simply calls these changes "New Light", how can you be sure they won't some day change their teachings on blood transfusions and refer to the change as "New Light" also?
When blood transfusions came in to use after WWII it was a pretty straightforward procedure. However, over the years numerous therapies and medicines have been developed that use blood fractions. So, it is not that the Watchtower has changed its policy, but rather, Jehovah's Witnesses are continually confronted with new situations and challenges. And because Jehovah's Witnesses look to the Governing Body of the Watchtower Society for guidance on these matters the leadership of Jehovah's Witnesses has issued its guidelines, basically ruling that using blood derivatives ought to be left up to the individual to decide. In other words, it is a personal matter. I doubt that will ever change.