For me, it's more about layers of intimacy. I don't have good words to describe my relationship to other people, so hierarchical terms seem easiest.
Fly is a primary. Moonlight and I just aren't at that level yet. Every day she becomes closer to what I think of as a primary partner - someone you make plans for a long-term future with, someone you enter into commitments with (like buying a house, raising a child), but I'm just not there yet. And I don't think it's appropriate or authentic of me to try to force equality, just for the sake of "everything's fair." Some of it is time-related. I've been with Fly for 7.5 years, Moonlight only a little more than 1. Fly and I have been through shit and come out the other side, and that has created a greater trust and closeness. Do I think Moonlight will become a primary partner? Definitely. But not yet.
Punk and I are secondary to each other, because that's how we choose to structure the relationship. Both of us have a lot going on, and we like having something loving but uncomplicated with each other.
To be clear, it's not as if this is how I describe or introduce my lovers. Fly is boyfriend, Moonlight is girlfriend, Punk is friend. It's just how I try to organize things in my head when I think about it, which is not very often. This also isn't about loving someone more or less than someone else, because I don't even understand how people measure love anyway. They're just words that try to encapsulate degrees of intimacy.
For the OP - if Fly and I broke up, Moonlight would not automatically become a primary partner for me, simply because that's not where our relationship is at currently. That doesn't mean it wouldn't happen, but I don't view my primary relationships as job positions that must be filled, especially not as a promotion.
If I were you, I'd hang back, give it some time, and see what happens naturally.