I don't know about other brokerage policies; well, I've heard things like RIchard says... boy, hope you're not going to his agency.
At Keller Williams, as an international brokerage (yes, in NY, too), specifies in the agent contract that you can and should take with you any listings or clients that are not under contract. (Unless the client has an objection, or unless the new brokerage does - theoretically, that's not likely.) If someone's under contract, then they can't switch, so you are supposed to pick an agent you trust, professionally explain to the client the need for the business transition (not the ugly details, but the general concept) and tell them you will be behind the scenes available to the agent who is taking over should there be any questions or concerns...oh, yeah, and split the commission as you agree with the agent that's taking over (subject to approval from the broker). Not sure where you're going, but wouldn't you LIKE to have this spelled out in the very beginning? Not only that, but if you are vested in the profit share program, it continues to pay you after you leave the brokerage, and pay your heirs after you're gone.
I have no clue about your area, but go to kw.com and look for an office near you - check them out. If there's not an office near you, think ahead and negotiate this with whatever brokerage you join. No matter where you go, eventually you will leave.