They probably can't change the license for WordPress. It's under the GPL and it looks like WordPress doesn't require a contributor agreement assigning copyright to someone else. They'd need to get every contributor to agree to relicense it.

There's also the question of who would relicense it. Automattic doesn't own the WordPress trademarks and if there were a contributor agreement, it would likely be assigning the code to the WordPress Foundation, not Automattic.

The WordPress Foundation non-profit doesn't have a good incentive to put a restrictive license on it. Part of the problem is that the WordPress Foundation doesn't seem to be truly independent of the for-profit Automattic. The point of creating a non-profit to hold the IP of an open source project is to prevent stuff like this from happening.