User blog comment:B214/General Discussion/@comment-27180505-20170331135923/@comment-30924344-20170401135957

Issei sacrificed his arm and at that time, he did it because he was really weak. We know that even now when Issei turns any body part into dragon scales, Rias or Akeno still have to suck the energy out for that part to return to normal. So the difference between V2 and V14 when Issei first turned a body part into dragon scales, is that in V2, Issei could not do that himself, but now from V14, he can, but the thing that stayed the same is the body part not returning to normal, although now sucking the energy out is only required once, whereas originally, Akeno and Rias had to keep doing it until V14. Issei needs sufficent demonic power to be able to transform his body parts back to normal himself and he will achieve this in the future as that is how he accomplishes transforming into a literal dragon. Now the difference between Issei with and without GR's body before, is that Issei would have had to ask Ddraig, [or for Ddraig to willingly do it himself], to gain dragonification whereas now he can do it manually because of his body being made out of a dragon. The similarity is both ways (Ddraig giving his 'flesh' to Issei) and Issei doing it himself, is there is still a dragon's flesh involved. So with that in mind, Ddraig totally has the ability to dragonify Issei as much as he wants, but Issei would need sufficent demonic power to be able to freely manipulate it back to his original body and this evident by how even now, he still can't do that even with the body of a dragon.

Ddraig does have the ability [and Issei could have used this], to dragonify Issei as much he wants, because if Issei can do that now, then so could have Ddraig as both way Issei has dragonified are done by the Dragons giving him their flesh/body. This means Vritra can do the same for Saji and Albion the same for Vali.