Earlier quoted context omitted.
I wonder, could OSM even change their licensing in that way if they wanted to? Because all the contributors submitted their content under the old license, they'd all have to agree to the new license again, right?
Contributions are under a contributor agreement, the OpenStreetMap Foundation controls the data and can change the license. The previous license change was a lesson learned.
> the OpenStreetMap Foundation ... can change the license.
Not exactly. The licence can only be changed to a "free and open licence" after a 2/3 vote of active mappers.
http://wiki.osmfoundation.org/wiki/License/Contributor_Terms