...what would happen to her if I did find a partner?
If you met another partner who was open to poly, then your current potential partner ought to be good with sharing you. Fairness is pretty important to poly relationships: if she gets other partners, then her partners have that possibility too.
If you mean that you would become monogamous, then it would be possible that she would be hurt. It's also possible that she would wish you well, hope for your happiness, and remain a friend. Isn't that true?
Why don't you ask her?!? It's great when people show they're thoughtful enough to consider their potential partner's future and their happiness!
Rather than over-think the issue on your own, I might tentatively suggest you open up to the girl and her primary. Ask them what they think, how they would feel if you became involved with them and then later fell in love with someone else.
I'm a secondary and my poly GF is very sweet about this issue. When I talk about meeting a particular woman and needing to manage the friendship so that no one gets unrealistic expectations and hence gets hurt, she says "Well, Don, but you can't control your feelings. If you fall in love with her you can't just shut that down." So what she's saying is, if I fall in love with someone else she will understand. Of course she is a rare gem
but really I think many poly people have that kind of understanding.
Don't burn your bridges before you even cross them! Good luck.