I don't know why people just can't accept the possibility that it was just meant to be that way from the beginning. The name of the first arc was Romance Dawn. Luffy set out from Dawn Island. The two prototypes to One Piece were also called Romance Dawn, and in one of the prototypes the Gomu Gomu no Mi was worth 5 billion berries, which means it was always meant to be a special fruit.
Romance is a narrative genre involving mystery, and adventure, that focuses on bravery and strong values.
Dawn is the rising of the Sun.
The first arc and two protorypes titles literally translate to "The Mystery/Adventure of the rising of the Sun".
Through this mysterious adventure the Straw Hats have gone on, Luffy became the Sun God. "The rising of the Sun God" He is the Sun who will bring light to the World
There is countless other connections that people have pointed out, but all of it just gets ignored, and people act like the only possibility is that it was a Retcon.
The issue here, is people wanted so bad for Luffy's connection to Joyboy to be something like reincarnation, so they could shit all over the generically overused plot point, and when it turned out to be connected to the Gomu Gomu no Mi, they needed something else to cry about, so "retcon" it is. Oda has always been a master at thinking outside the box, so taking the tired reincarnation trope, and turning it on it's head, by having Luffy connected to a figure from the past via his Devil Fruit is 100 percent something he would have come up with a long time ago when thinking about how his main protagonist will progress through the story.
Added Bonus:
On Alabasta Crocodile hinted at Devil Fruit awakenings by talking about completely Mastering a devil fruit, so there were plans for Devil Fruits to be able to be evolved way back in the beginning of the story