[user34283]

Afaik you can do whatever you like to GPL licensed code, you do not need a license to refactor it.

I understand you need to publish the source code of your modifications, if you distribute them outside of your company.

[skeledrew]

You can do anything except change the license, which ensures that right to do anything passes on to others in perpetuity. That's how it's designed.

[duskdozer]

You also can't relicense it to be less restrictive

[pocksuppet]

Or more restrictive! There are certain exceptions permitting combinations of open-source code however.