I think actually you can. If you rewrite 75%? 80%? You can relicense it. Legally I think the law views that as new code not a modification. A lawyer may need to correct me but I seem to recall something like this.
Most certainly not. I recall that, in the SCO vs. IBM case, they discussed copyright of individual code snippets, on the range of a dozen lines of code, that were part of a bigger codebase.