There's no one right way to become happily nonmonogamous—and there's more than one wrong way to do it, too. So here's Flawed Polyamory Strategy Number 11: Evolution Theory.
Chris and Pat fall in love. Along the way, having an open relationship is discussed. Chris says, "Okay, but I want to be monogamous for a while. Just until we feel secure in our relationship." Pat agrees to this. Time passes, and Pat feels attracted to other people. However, because of the agreement, Pat doesn't act on the feelings or tell Chris about them. Then one day, Pat meets someone and thinks, "Wow." Pat says to Chris, "Remember how we said we'd open up our relationship eventually? I think it's time."
Chris knows Pat is bringing this up because he's attracted to a particular person. What usually happens now: Chris gets upset. Yes, Chris agreed to maybe opening the relationship someday. But if Pat wants to change their comfortable state of monogamy right now, then he must be head over heels about this interloper. That makes Chris feel insecure. Therefore, Chris says, their relationship is clearly not ready to be open.
Potential outcomes for this bad situation? Best case: Pat gets Chris's reluctant permission to pursue the new person, but only after lengthy, painful emotional processing. Second best: Pat resentfully agrees to continue being monogamous, but they both feel lingering hurt, anger, and loss of trust. Worst case: all-out war, resulting in Pat leaving Chris—not for the new person, but because the fighting has soured their relationship.
The monogamy-now/polyamory-later trap seems seductively reasonable, and I've seen many well-intentioned people fall into it. But this theory of evolution is a myth. The idea that one should start a relationship by being monogamous and then "evolve" into polyamory is flawed because polyamory is not inherently more (or less) evolved than monogamy. It's simply different. All relationships change over time, but no matter what promises are made, changing one's long-term relationship status from "monogamous" to "otherwise" is a transition rarely achieved with ease.
If you know you want an open relationship, stick to that from the beginning. Make agreements about it while it's still an abstract concept and you're both so silly in love that anything feels possible. It won't be an easy discussion, even so. But if you wait, the scared partner tends to think, "This is all That Other Person's fault. Everything was fine until she/he came along!" That's neither accurate nor helpful.
However, a poly-confident partner should not cruelly disregard a nervous lover's feelings. Here's my advice: Be honest about how you'd like your open relationship to look and then exercise only a fraction of the consented-to latitude at first, until your partner learns to trust you. Once a relationship is closed, it's hard to reopen. But if it stays open, and adjusting it becomes not a matter of if but how, then your relationship has a better chance to evolve and flourish.