Bear already mention this in his post

> In addition to the above mats:
Exchange either T3 weapon (Must be SL20 and MLB) for T4 Soul Weapon. Note that the T4 Element Dmg Weapon requires Element Dmg for exchange, and the HP one requires HP. You cannot exchange say an HP weapon for an Element Dmg Weapon. Just think of it as a direct FLB upgrade of the T3 weapon itself.

It's like a FLB, which means you still keep +99. But unlike FLB skill lv is reset to 1