Without a CLA or equivalent mechanism they would need to get the agreement of every contributor to change licences. Once a project gets large enough the sheer number of people involved usually makes this impossible
0-BSD doesn't have any conditions at all. 2-BSD does.
The MIT → AGPL example isn't very illustrative. The requirements under AGPL are a strict superset of the requirements under MIT. Going MIT → 0-BSD is a bit trickier, because distribution under 0-BSD doesn't meet the requirements of MIT.
Here's an example where an MIT-licensed project switched to APGL recently without needing to contact contributors[0].
Interestingly, the MIT license explicitly names and allows sublicensing, but the 2-BSD and 0-BSD do not call it out explicitly.
tldrlegal[1] seems to think 0-BSD allows sublicensing, but doesn't mention it on the page for 2-BSD[2]. I'm not sure what to make of that.
[0]: https://github.com/immich-app/immich/discussions/7023#discus... [1]: https://www.tldrlegal.com/license/bsd-0-clause-license [2]: https://www.tldrlegal.com/license/bsd-2-clause-license-freeb...