Relicensing
Changing the license of an existing project, which requires consent from every single copyright holder – or a CLA that already granted it.
For a community project it is painstaking work. Mozilla spent years tracking down contributors before the codebase could be tri-licensed in 2004, and VLC contacted hundreds of developers to move libVLC from GPL to LGPL around 2011. OpenStreetMap deleted data from contributors who would not agree to the switch to the ODbL in 2012. The Linux kernel can realistically never change license; nobody even knows how many of its authors are still alive.
For a company that owns all the code it takes an afternoon. Elastic went from Apache 2.0 to SSPL in 2021, HashiCorp from MPL to BSL in 2023 and Redis from BSD to a source-available pair in 2024. All three got a fork for their trouble – OpenSearch, OpenTofu, Valkey – and both Elastic and Redis later added the AGPL as an option.