Embercore
As Belialthorn pointed out to Kat in Twilight. Most of his previous spells had been included in his new rank spells, including [Flametongue] but the formation had also changed, which meant he would have to adjust all the previous formations he learned.
This was one of the reasons some wielders would rather concentrate on new spells for their new ranks. Or adjust their arcane flow to fit the previous formation.
Neither was easy but discarding previous spells was more easier because it provides you with time to focus on new spells. It also left you open for attacks.
Which was why some hide and train in secrecy when they gain new ranks. Belialthorn's case was even more unique, he had jumped a whole rank to that of Errant.
He paid for it with a splitting headache as he tried to learn the core spell of his Errant rank— Embercore. Which will turn the body into solid fire.