But, but, but... I can show them how fantastic poly is! Sure, they _say_ they're mono, but maybe they'd be open to poly, if they really like me. Maybe we can do mono/poly! Sure, that's it, there's my out! I'll be poly, and they can keep being mono, and we'll have this great wonderful relationship. It'll all work out, if they like me enough to give this a shot.
Um, this is like taking a straight person and making them gay in just this one case. If someone is in a relationship with a poly person just to be with that specific person, chances are really good that there is some level of coercion being felt.
The mono/poly situations I've seen that are working well involve a mono person that is poly in almost every way as far as their approach to relationships, excepting the application of addition people on a personal level. They see the ways that being with a poly partner benefits them personally, and aren't doing it to please anyone.
On the other hand, let's say you meet this great person from the first paragraph, as you are getting to know them (but before you are actually in a relationship with them!) you share that you are polyamorous. They ask what that is, they see the relationships you already have (if you are currently partnered), they seem to show interest on a personally applicable level, as though this is something that they've been wanting, but never thought possible. You may just have something here!
This is about how the relationship with S began. He knew us, knew about our relationship style, and sure, the motivation to be with me was a biggie, but polyamory, as a philosophy or set of ideals, had personal resonance for him. He'd also decided that monogamy wasn't a relationship model that worked for him.
He read, he asked questions, he exposed himself to things that were outside his previous experience, he acknowledged it when he made mistakes, corrected, and moved forward. Yep, I knew there was a learning curve to get through, and while I was willing to support and encourage, there was a lot that he had to do for, and by, himself.
Conversion from mono to poly is possible. Almost all of us made that transition, but the key factor remains personal choice. Going from mono to poly to be with someone specific is like losing weight to please another. The pounds inevitably come back, because the change wasn't motivated for, and by, self. The change must come from the convert, and not the converter.