User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-112.198.164.172-20120603055106/@comment-68.234.125.79-20120603195447

It's already been stated that Issei is already at the level of a High Class Devil, but that some of the Purebloods didn't want him to jump from Low to High Class so quickly, so they just asked him to wait a few months or so before they gave him the opportunity to promote again. Saving the Underworld probably just lowered the amount of time they would need to wait between promotions.