I have been with Pengrah 10ish years and have been open from the start. We were open because it was a long distance relationship to start... we never bothered to close. And turned into unicorn hunters, relatively successful ones too.
Most of the people I have encountered have been the 7 year itch open relationships. There are exceptions, including the relationship with Sourgirl I am in now.
I am not sure what it would be like to go from pure monogamy to being non-monogamous. The amount of expectation in monogamy is
So I guess our version of being married and finding poly fits a little different than what you are asking about. I haven't been purely monogamous since I was 21. The sense of being owned sucked the wind out of life for me.