As i Said before and i Still think is like that:
Joyboy ISNT a Power up, isnt haki, a Holy Spirit, a posession, Power or anything.
Its a title, NOTHING Else.
The PU is luffy awakening his fruit. He being called Joyboy at the same time he awakened his DF by Zunisha is probably because luffy achieved a mental state that made him able to awake his fruit and also laugh at the situation, and that state of spirit and mind made him a equal to a previous Joyboy on Zunisha eyes.
I dont think oda Will go at ALL to joyboy changing luffy, i think joyboy was always luffy for who he always been, but became more clear now after he Achieved this mind state.