"Honey-I don't ever want to lose the closeness we share, and I think honesty and trust are critical components of that closeness. Could you please read this article on polyamory so we can discuss any questions you may have about the life changes I'm making?"
If you are already close-there shouldn't be too much difficulty in accepting change-sounds like a lot has happened.
But I find that when someone potentially surprising is coming up, it's sometimes nice for it to be in writing so that I can mull it over on my time and then come back to the table for discussion, when I'm ready. Opposed to feeling like I am "on the spot" with that surprise.
"Love As Thou Wilt"