Possible conversation that could happen:
Secondary says: "I still wants us to continue our romantic relationship, even though I am dating Newbie. Our relationship is still fulfilling for me.
You say: "Great! I'm happy you have Newbie. But I like open, honest relating. So... until further notice,could I assume and expect that we are "broken up" or "on ice" until you tell me that you let her know? And I can give her a call to say howdy and check in? Or she calls me to do same?
Secondary says: "Newbie is open-minded. I would be surprised if she did not approve, especially since this is a romantic rather than sexual relationship, not to mention long-distance."
You say: "Great! So can I expect you to let me know when I can call her? Or expect you to tell me you let her know and you asked her to call me by ___? So I can expect a call from her?
We don't have to be best friends or anything but I'd appreciate a courtesy call since the polyship shape is changing shape. I like knowing who everyone is the poly network and we are all clear to continue and set to exchange labs and things for sex health reasons from time to time."
It isn't unreasonable to ask.
Then ball's in his court. He knows what you expect. If he agrees you can expect this behavior from him? Then he either delivers or not.
It's ok to feel scared. The 3 people thing could be ENDING. The new 4 people could STARTING. The "in between" transitional space.... not sure space? That space is weird -- neither here nor there. Just kinda up in the air... until you land.
So could align behavior toward a good landing. Letting Newbie know he comes as part of a poly package -- well, being honest and forthcoming so Newbie can make informed decisions is treating everyone respectfully, right?
ETA: Sounds like you sorted it out. Kudos!