I have some good friends that transitioned from a "primary" to a "non-primary" relationship. The stresses of trying to merge their lives seem to make it harder for them to love one another because their lives weren't that compatible.
I haven't talked at huge length with them about it but one thing they did say that helped them in their own transition was to not jump right into finding other primaries right away. The moved into separate living quarters and proceeded to "date" each other from their new, non-merged position for quite a few months before they started seeking new relationships. They said it helped them reset their relationship on this new setting but keep some security.
That was quite a few years ago, and they're both still together with other primaries now.