A great question that I think others have probably answered more eloquently, but essentially this movie and the characters have been described as the growing pains of adolescence to a large extent. And part of going through adolescence is what’s called the ‘sexual awakening’ or just growth and awareness in general.

So for Rey, the hairstyle changed when she made that decision – to go against what Luke said (don’t go to the Dark/cave thing) and to find her own answers. So she dives in, her hair changing – what was also the catalyst for this decision? Her discussion with Kylo – shirtless scene – and his talk about the past and becoming what you’re meant to be.

Rey & Kylo’s story is a sexual awakening for many reasons, and it plays into the hair change that happens immediately afterward.

So the moment she dove into the pool, it signaled the beginning of her taking autonomy and ownership of her self discovery and identity – as seen in the mirror cave, where it basically tells her ‘you already know the truth, your identity is forged by you, no one has answers for you’ – aside from the stuff about how it also points to Ben Solo being her future.

So that’s pretty much it, it’s signifying the change and growth from young adolescent to perhaps that burgeoning adulthood.