If you are happy with your agents marketing then you should transfer with them. If you are not happy with your agent you can speak with the previous Broker about being assigned to another agent within the original brokerage. Best of luck for a speedy and profitable sale!
In most cases, agents are independently employed-the brokerage is only a place to hold the license by law, so it is the agent that gets the job done for a seller. It is their marketing and sales expertise that is needed to get the home sold and make sure that the seller is protected through the entire process.
You hired your agent for a reason, whatever that reason is, has it changed because he now has a different company name on his business card? If you are happy with the service you have received and have a good relationship with your agent, stay with them. They already know everything about your home and situation, why would you want to bring a new person up to speed?
Best wishes on the sale of your home.
Generally people list with an agent, because that is the best person for the job, and the company they work for is not the reason. If that is the case, then this should be good for you. Ensure, though, that your listing agreement with the original brokerage gives you that quick "out" -- I am sure your agent has made sure you are covered -- but make sure to ask him or her about it!
On the plus side, your home will come through the Multiple Listing Service as a brand new listing, and a new sign will go in out front -- which will create a bit of a buzz around your house! New marketing, a fresh start -- all with the agent you trusted in the first place. Sounds like a win-win situation to me!
Good luck to you!