My friend, who breeds TBs for eventing took a tour a few years ago of Hanoverian breeding farms in Germany, and one of the farms on the tour WAS breeding TBs to TBs and branding them Hanoverian. The guide told them it was a special farm to breed eventers. She was quite shocked as she knew of the 50% rule.
The 50% blood rule would apply to, say, breeding a Hanoverian approved TB mare to Sandro Hit (at least, I only counted 6 Hanoverians in his 4th generation - so 37.5% Hanoverian blood). According to the blood rule, a foal from this cross could not be registered as Hanoverian because, even though both parents are approved for Hanoverian breeding, neither parent is REGISTERED as a Hanoverian, and neither parent possesses at least 50% Hanoverian blood.
Another example is breeding that same Hanoverian-approved TB mare to Royal Diamond. He is registered as Oldenburg, and approved for Hanoverian breeding, but there are NO Hanoverians in his 4th generation (some in his 5th generation, but they only contributed 18.75% Hanoverian blood to his pedigree). So again, since neither parent is registered as Hanoverian, and neither parent has at least 50% Hanoverian blood, the foal could not be registered as Hanoverian.
The NEW ruling would make it possible to breed that TB mare to either of those stallions and receive Hanoverian papers for the foal. And my guess is the reason behind this change in policy is because the Hanoverian Verband is seeking to expand its reach even further (as mentioned, it already absorbed Hesse a few years ago), and this move would help attract breeders into the Hanoverian fold who may be taking their foals to other registries.