I think it is harder for mono people because mono is everywhere, it's expected, and they might not even know about poly until their partner talks about it.
I think most of the time, the poly partner is the one who tries to be mono, because that's "the way things are". I don't think many poly people manage to face their spouses and be open about who they are, how they work, and what they need. I would bet that cheating is much more common than honesty, at the very least, and I wouldn't be surprised is polys who stay mono for their partners were also more common than polys who are poly while in a relationship with monos. (I think more polys date other polys. It's just simpler).
"Who is expected to change", you ask, but the only time a partner is expected to change is when a poly has to be mono. Polys usually don't force monos to become poly and date other people. What changes isn't the mono person. What changes is what they can expect and require from their partner.
In my opinion, it's a bit different, because polys who are with a mono also have to deal with them differently than with a poly partner. They have to dedicate more time and attention to them because they're their only partner. They have to be as comprehensive as they can about their being mono. They have to try and understand their point of view, understand the boundaries they do have, etc.
Often, you will end up with a compromise where both people have to adapt to the relationship. The poly person might find they have a lot more limitations than they did/do with poly partners. The mono person, of course, will have to deal with their partner having other people in their lives. It's undoubtedly hard.
But I want to state my poly point of view here: a mono partner will tell me they have a say not just in the relationship we have together (which is self-evident to me) but in any relationship I might have. Ideally, they want to restrict them to none at all, which is hard for me to wrap my mind around, since my desire to have several partners doesn't affect in any way relationships my mono partner might have outside of me. To me, being poly affects my relationship with my partner to some extent, but leave the rest of his life his own. My partner being mono and wanting me to be mono affects my life even outside of him, outside of the range of our relationship. It feels much more intrusive to me.
In a successful mono/poly relationship, I feel that both partners are themselves. The mono partner is mono, the poly partner is poly. Your question seems to phrase it from a mono point of view: if the mono partner is mono and the poly partner is mono, the poly partner changes, and if the mono partner is mono and the poly partner is poly, the mono partner changes.
That's not true, in my opinion. A real equivalent to a poly person having to be mono would be a mono person having to be poly. My being poly doesn't change my partner's identity as a mono person. They have to make concessions and so on, but I strongly feel it's not the same as requiring me to be mono.
This being said, I do feel that requiring someone to be poly is worse than requiring them to be mono. I feel that when one person wants something and the other person (directly involved) doesn't, the person who doesn't want it should be the one who "wins". I think so for sex (so one person stays frustrated or has sex elsewhere instead of one person raping the other) for children (no forcing a child on someone who doesn't want to be a parents, do without or have a child with someone else) and so on, so I would also say, no forcing partners on someone.
So if the choice was between forcing the mono partner to become poly and forcing the poly partner to become mono, I definitely think there is less harm in forcing the poly partner to become mono (or leave the relationship).
However that's not what we're talking about here. We're talking about letting each partner be the way they are, and having to adapt to the other being different. The mono partner needs to walk more than halfway, but that's only because the poly partner has had to adapt to mono lifestyles all of their life already, so they've already walked part of the journey towards middle ground.