For the license to be changed every team member needs to submit a written agreement that he agrees to the change, otherwise their contributions must be removed as they were written under a different license, the only exception is usually permissive licenses such as MIT/BSD 3 clause.
Usually, to rugpull FOSS contributors, companies who maintain FOSS software ask contributors to sign a CLA which waives their rights and lets the control their contributions. Immich isn’t doing any of that, and it will likely remain AGPL forever because changing the license will be a big hassle for them with the amount of contributors.