Advertising is a business, and hey, we want to stay in business. right? This is why I can't stress enough, protect yourself and your agency. Make sure you have a contract in place. Things change. It's inevitable, it's going to happen, if things are in writing, your can make it out of sticky issues a lot easier. Especially on project and production work, treat any and all scopes of work as a real, honest contract.
In writing, set clear definitions, clear expectations, explicitly stated and agreed to by both parties in a contract. Make sure the client and you understand what everyone is agreeing to in a written down and dated, signed contract.
If goals change, beyond minor things - stop the job. I mean it, the old job is done, over, kaput. Now it's a new project, the original contract is over (that jobs has ended), time for you to have them sign a new one. Most importantly, don't just grab things from here or the internet, get real about it.
Go find yourself a good lawyer, they make you money and they save you money. You need to protect yourself and your specific needs and requirements, so you can have a good contract in place, reviewed by a lawyer.
Besides, contracts aren't evil, they are just necessary in the business world. And contracts go both ways, protecting both you and the client.
Don't start work without a signed contract.
Don't blindly accept their terms. Negotiate, make it fair.
Don't back down on the important stuff;
Make sure you own everything till final payment (Upon full payment...), make sure you have a termination or kill fee in the contract & review all liability.
Be specific and confident about money & terms.
It's all about relationships. At the end of the day, they can fire you, and you can fire them. When everyone is on the same page, things get less messy if things go south if both sides are in agreement. Happier you, happier them.