There were some bishops who left with Luther, and a few who ended up so by default.
As the BXVI and the papal primacy, the Pope can’t change it. To do so would bring a whole lot of the post Schism doctrines into play, and would probably set off a lot of other schism in the RCC.
Do you have any references/names?
Anyway, if they did join Luther, then they would have taken the helm of this new church, not Luther. Obviously, part of Luther's dissention was dissolution of the ordained order, which makes any bishop(s) joining Luther meaningless.
After anarchy that followed, the Lutherans gathered their wits (unlike the Calvinist and Zwiglian followers) and tried to re-establish a church. But by that time everything has been broken.