DH and I talked for hours, over weeks, about this, and came to a tentatively functional way of going about it, figuring out how this transition into the "not-monogamous" world would work.
She reacted as if I had come out to her as a conservative or something.
Way bigger of a deal to her than I'd have expected, given that she's openly and totally poly. I thought our issues were more my fear and need for information and answers and her refusal/inability to give me more than "not yes and not no" when I was reeling. We sorted that, after a few rough conversations (one which resulted in screaming).
I think DH and I are doing pretty well, as we talk about it a lot. We're even looking at seeing a poly-friendly therapist and making sure we're doing alright, as we have some downright amazing communications skills (hard-learned over the last 15 years) and there isn't a single thing I feel I can't talk to him about.