You have a license that allows you to make changes to the code, as well as distribute and sell modified versions of that code, such as the Apache License or MIT license.
The author of the software did not assign the copyright to you.
Simply put: would the original developer still have some significant copyright left on the matter?
Yes. What ...
This depends on the license you buy. Your license might preclude you from changing any of the code or force you to attribute the original code maker even if you change 99.9 percent of the code. And it might deny you the right to resell what you did with the licensed code.
Get a lawyer and read the license.