my husband and I have been in a similar situation to yours when we started to discuss the poly topic. And you don't have to be totally OK with that. This will take time and that is something both of you should be aware of. The analogy she used (gay/straight) is something that is kind of a hot topic around here, my take on it that it is 100% true, I couldn't bring myself to not love someone besides my husband, it was just the way I worked. But that is no excuse for getting things started in a rush. She should be aware of that.
There are great threads around here that deal with lessons others have learned, mistakes that were made while opening up a former monogamous relationship and the difficulties of a mono/poly relationship. I would suggest both of you start reading a bit about what to expect. That helped me big time when my husband, boyfriend and I started to give poly a try. But one thing you can be sure of: Any problem you may have, be it as tiny as you can possibly imagine, will surface while facing poly. I am a bit worried about the cheating going on in earlier times of your relationship, is that topic solved and healed already? If not, it will become a problem again.
It's a lot of work and you are right to ask what is in it for you. It should be the thing, that is in every relationship that makes you happy: Being with the person you love. As you have correctly stated, you shouldn't want to change that person, because she will become someone different by that, and that wouldn't feel right. As far as the mono perspective goes, my men are both mono and they told me that they don't think too much about the other, they are just happy being with me and that what counts. If you are able to reach that place, everything will be good, but the initial phase will be rough, because it is just new and you need to get used to that. Our life has calmed down quite a bit, but there have been rough times.
There was some adjustment work to do from my perspective (the poly one) as well, it isn't easy to change your relationship style if all you ever learned is to do relationships like all the other people around you, meaning monogamous living-happily-with-THE-ONE-ever after. This is what takes time; accepting that there are other ways of living out there and testing out if one of those could be made for you. Everything that could make you reach a comfortable place will only come with time and experience.
Good luck on your way
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.