> Because Wikipedia was under a Creative Commons license, anyone who didn’t like the way the project was run could copy it and start their own, as a group of Spanish users did when the possibility of running ads was raised in 2002.

Correction on this: Wikipedia was GFDL until 2009. See https://en.wikipedia.org/wiki/Wikipedia:Licensing_update .

If I remember correctly, the licence change was achieved by the FSF releasing a special version of the GFDL allowing the change, taking advantage of the "any later version" part of the original licence.

OpenStreetMap's licence change was much more difficult. Agreement had to be sought from all editors and for the few that didn't respond their work was removed. We actually replaced the work of most non-responders before the licence change though.