How does this work for people who contributed code under one license (thinking it would stay that way) and don't want their contributions released under another license? In practice, I doubt they'll run in to any issues, but what if they wanted to release all the code under a more commercially permissive license and contributors had a problem with that?
Does MAME own all contributions? Could they change the license to require license fees? If not, why can they change the license after the fact?
Answering to "why can they change the license after the fact?" , I think the answer is something close to "Licenses can be changed after the fact, provided the owners/maintainers get permission from each contributor, or rewrite the problematic parts".
VLC went through a similar transition (from GPL to LGPL); jbkempf's blogposts about this experience may be of interest to you:
I don't know what they are doing for people who are no longer reachable online (deaths, disappearances, etc); but in my case, they reached out to me and asked for (and received) my permission.
Others below mentioned that they are getting permission from their contributors. I guess they checked beforehand how the bulk of them feels to make sure there is not to much they have to rip out and replace if someone objects.
Does MAME own all contributions? Could they change the license to require license fees? If not, why can they change the license after the fact?