IANAL, but in the US some derivative works based on material still in copyright are considered infringing. For example, taking Microsoft’s source code for Windows, adding some functionality, and selling it as your own OS would be infringing.
a derivative work must be different enough from the original to be regarded as a new work or must contain a substantial amount of new material. Making minor changes or additions of little substance to a preexisting work will not qualify the work as a new version for copyright purposes.
Edit: Outside the US, of course, the rules may vary.
IANAL, but in the US some derivative works based on material still in copyright are considered infringing. For example, taking Microsoft’s source code for Windows, adding some functionality, and selling it as your own OS would be infringing.
You’re right—according to this PDF on copyright.gov, the requirement is that
Edit: Outside the US, of course, the rules may vary.