If Leviathn is smart which I know he is, then he will never fly under that name again. It is immortalized, and there is nothing he can ever do again to touch a flame to the reputation he has built since he quit.
I'm not sure what reputation I've built since I quit, but I felt I built a pretty decent one while I played.
If I were to fly again regularly, I'd use the Leviathn handle, but I'd probably avoid it initially while getting my game legs back under me. As fun as flying shades can be at times (Leviathn was actually a shade of Calamari, which was a shade of DMF, my original handle), ultimately folks figure it out when you have a pretty obvious flying style. I knew Nomak was Duckett after dueling him a couple of times way back in the day because he had pretty obvious dueling proclivities. A rose by any other name should smell so sweet.