- 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 you've created is a derivative work. US Copyright Office Circular 14 gives "[a] revision of a website" as an example of a derivative work. It also notes:
The copyright in a derivative work covers only the additions, changes, or other new material appearing for the first time in the work. Protection does not extend to any preexisting material, that is, previously published or previously regis-tered works or works in the public domain or owned by a third party.
So while you would own the copyright to any changes you made to the software, the original author would still own the copyright for the 40% of the original software that remains. What that allows you to do with the combined work would be dictated by the license under which you obtained the software in the first place.