Just a thought
This explains few things which I was unable to understand
1) unconscious luffy staring at kaido
2) Luffy laughing while fighting kaido which we all found weird.
And, I read here that someone said luffy's personality will be affected.
Well, you will never no.
First luffy is always smiling and enjoying like he is doing in Gear 5.
One can say that Nika nika no mi affected luffy personality the moment he ate it.
So, luffy won't change because not only Nika is like luffy but if any change happened then it happened when luffy ate this df and not now
Second, This is a transformation and not reincarnation.
If anything Luffy is joyboy not because he was reincarnated but because he has nika df.