If I have a piece of software which is licensed under the GPL 2.0 only (there is no "or later" clause in it's license), and I modify it, can I relicense my derivative work as GPL 2.0 or later?
It is clear that I can't just change the license and call that a derivative work, but beyond that, when I really do have a derivative work, it is not clear that I can change the license of the derivative work.
The GPL requires that derivative works as a whole have to keep the same license (with some exceptions, e.g. GPL → AGPL). Thus, a modified version of a GPLv2-only program cannot offer GPLv2-or-later. That would provide additional permissions regarding the original portions, but you're not authorized to offer these permissions because you're not a copyright holder for those portions but are bound by the GPLv2-only license.
However, if you create a component that is not derivative of the original software, you could offer that component under any license you want. But when you combine the component with the original software, the resulting work as a whole would have to be GPLv2-only. Thus, your component should have a compatible license, e.g. GPLv2-or-later.
