There are two kinds of code here: Your code, and someone else's code.
The code that YOU wrote is yours. You can mark it any way you want.
The code that someone ELSE wrote, that you included in your project, is NOT yours. This includes code that you modified, as opposed to originated. You got that code under the terms of their license, presumably BSD, and you are required by law to continue to observe the terms of their license.
Among other things, that means that you can't change the copyright notice and license on their code.
Having said all that, here's the ground reality. There exist scumbags. The scumbags will grab your code, regardless of your notices, remove the notices, file off the serial numbers, and do what they want. When and if you discover what one scumbag did, you will have the opportunity to spend thousands or tens of thousands of dollars going after him. While you are going after him, fifty other scumbags will be doing the same thing.