I too am familiar with the concepts of rules and boundaries that no longer apply. I've gone from being uncomfortable with my bf touching his other gf in front of me to comfortably watching them kiss and snuggle. I've gone from being triggered while hearing about their sex life to curious and kind of turned on.
Basically, when my bf and I started out in this open relationship, I was very scared of being replaced. We kind of had an understanding that we were one another's primary partner. Now that he has another long term partner outside of our relationship and so do I, the concept of Primary/secondary has completely dissolved.