My familiarity with disgaea is very limited. But I will say this.
Perhaps she fell -because- she felt sexual desires and had sexual experiences. Therefore, assuming that the process is ruled by something remotely even a smidge logical, there is a fair chance that she fell directly from beautiful angel into succubus.
In the Disgaea setting, there is a reincarnation cycle. There is also a way for living creatures to bypass the reincarnation cycle, by going through the Infernal Bureaucracy and spending the appropriate amount of Mana (units of mystical energy; more = stronger). This process is known as Transmigration.
Spoilers abound, but suffice it to say: Flonne is instantly transmigrated into her Fallen Angel form as "punishment" (actually a reward, allowing her to continue being near Laharl and Etna - something she couldn't continue doing indefinitely as an Angel). There are Succubus monsters available for Transmigration, but Flonne shows no similarity to them at all, aside from the tail and wings (which nearly all demons have in Disgaea). The only real shift in personality she gets is that she lets her less-than-nice side out a little more often, which is most likely due to the social influence of the residents of Makai.
