I have software licensed under GPLv2, which I would like to relicense under GPLv2+. Unfortunately, I an unable to contact some of the contributors. Can I proceed, or am I stuck?
Since licensing can only be done by the copyright holder, you generally need individual permission from each copyright holder to change the license of their code. (If the project is being re-licensed to the GPL from a compatible license, then no re-licensing from the copyright holder is necessary; however, versions 2 and 3 of the GPL are not compatible with one another.)
Ways around this are:
Remove or replace all code written by the unreachable contributors.
Have contributors assign copyright to a person or legal entity that controls the project, so that there is exactly one copyright holder. (This is a preventive measure. If you have not done this already, it's too late when you cannot track down a contributor.)
But if you can't do either of those things, what do you do?
Historically, re-licensing of software without explicit approval from all contributors has been done before, under the advice of legal professionals. However, the legal mechanics of it are unclear and have never been tested in court. The two prominent examples appear to be:
Mozilla relicensed from the MPL to a triple-licensed MPL/GPL/LGPL scheme
The Dolphin emulator relicensed from GPLv2 to GPLv2+.
The Dolphin project produced a report about the process, but it lacks any specifics about how the legal mechanics of the switch work. They say (emphasis mine):
Out of all [200 contributors], we managed to contact all or take care of the code of all but 10 of those developers. Considering that many of those accounts are now dead and the people seemingly vanished from the Internet, finding the rest is akin to squeezing blood out of a stone.
That's when research provided us some much needed relief when it seemed as though relicensing was an impossibility. In 2003, Free Software lawyers consulted for Mozilla's relicensing project and stated that relicensing with the permission of just 95% of contributors was fine, as long as there were no objections in the remaining 5%.
[...] Instead of simply asking all of our active developers and relicensing, we made a very strong effort to get a hold of every single developer. Despite this, it simply wasn't feasible to make contact with every single person who worked on Dolphin.
[...] We definitely would love to hear support from those that we were unable to contact, and if there are any concerns we will gladly address them in a prompt fashion.
That posts links to a post from Ciaran O'Riordan:
Someone who works with many lawyers on free software copyright issues later told me that it is not necessary to get permission from 100% of the copyright holders. It would suffice if there was permission from the copyright holders of 95% of the source code and no objections from the holders of the other 5%. This, I’m told, is how Mozilla was able to relicense to the GPL in 2003 despite years of community contributions.
There is no information available about why Mozilla's lawyers believed it was acceptable to re-license without each copyright holder's permission. It's also unclear if any factors apply to Mozilla specifically that might apply less strongly to anyone else in a similar situation.
In sum, we know only that:
You need the permission of every contributing copyright holder to re-license a work
Mozilla's lawyers believed (for unspecified reasons) it would be acceptable for Mozilla to make a good-faith effort to contract all contributors and then re-license, even if 5% of the contributors were unreachable (and none of the contributors disapproved).