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.
Contribution/contributor license agreement. It's a document that transfers the copyright from the original author - developer submitting a patch or PR - to the project owner, e.g. FUTO. If FUTO required CLA for all Immich contributions, then FUTO would own the copyright for all the source code of Immich. This allows FUTO to relicense Immich under a different license, other than GPL, for whatever purpose, without asking anyone. For example they could make modified Immich versions for sale, or sell the Immich source code to third parties under EULA or any other license. Without a CLA, FUTO would have to get written agreement from every Immich source code contributor to change the Immich license, which would happen in 2000 and never, at least not without ponying up cash.