Thanks for these questions - clearly I'm at the beginning of this thought process. So far the primary-ness has been emotional and practical (as you write - shared finances, living spaces, etc.). I guess I see the change as living independently, being financially independent, and no longer necessarily having emotional primacy. I'm not sure how to change the practical parts without impacting the emotional parts - but I like the suggestion in your question that it's worth separating the two parts.
Given that family/children is what I want, either another primary partner or a different primary partner seems like the direction I want to take. Right now, I'd like to negotiate new partners with my husband (as we've done so far), but I imagine that might change.
Thank you again for writing - I really had to parse out my thinking. It's such a change and I'm so grateful for your input.