Lineage 2 Tower Forum

I have a dumb q about pm.Some months ago I spek on pm whit some1 whit(lets say) x nick name on forum.I whant make some trades whit him. After some time I receve a pm from another nickname lets say Y.But my all old send pm to x,look like I send to Y. I see that on my outbox.
I ask Y if he is x and change username,the answere whass NO. How is possible this?

Yes username change is possibile