User blog comment:B214/General Discussion/@comment-27180505-20170331135923/@comment-30924344-20170331223445

Well since Issei did have his left arm transformed into a dragon arm, that means Ddraig could have granted him the ability to transform any body part, but Issei never thought about that potential and it does not matter now, because his new body made of GR's flesh allows Isseii to do that anyway manually and of course it's better since it allows him in the future to shapeshit into a massive dragon. From that, Ddraig himself could have granted Issei the ability to transform into a dragon, but not as massive and powerful as GR's version.