My partner said she was okay with polyamory when we got together, but is now upset about it

So I’m currently in a long distance relationship with someone who said she was okay with me being polyam from the first date (even before the first date I told her). Now I’m on tour with my band for 6 months and I promised to be monogamous until I got home and we reconnected. I’m 2 months into the tour and she is already freaking out about me wanting to date other people when I get home. I still have a little over 4 months on the road and I'm not really sure how to handle this. 

It sounds like she's not actually okay with you being polyam. If she doesn't want you to see other people while you're on tour, and she is already nervous about you seeing other people when you're not on tour, the issue is that she's threatened by you seeing other people.

Or, it could be that she is feeling insecure because you're gone and she feels like you two didn't talk it out enough before you left, and her wanting to continue the conversation feels to you like "freaking out." 

Either way, I think you're getting distracted by red herrings in your situation. The band tour doesn't really matter; the promise to be monogamous during the tour doesn't matter; her insistence since day one that she's okay with polyamory doesn't matter. What matters is that she is upset and threatened by the idea of you dating other people. That's what needs to be addressed.

You need to identify and clarify your expectations and needs and boundaries with her. "Part of dating me is polyamory. I won't be in a relationship where I can't date other people. If that's not something you're comfortable with, this won't work out." If she insists that she really is okay with you dating other people as long as specific concerns are addressed, ask her to clarify for you what those are and work on a plan to address them.

If she can't - if it seems like she's just trying to argue herself into being able to date you, or that she's assuming that her future self will be okay with something her present self clearly isn't - it's probably best to end things. Don't speak for her, by saying "you say you're okay with this, but you're really not, and I know what's best for you better than you, so I'm going to end a relationship that you want to continue, for your own good." That's never a good way to end things. Instead, frame it as you not being happy in this arrangement, getting the sense that polyamory isn't working between you two, that you aren't able or willing to provide all the emotional baby steps it will take.