"You can't relicense something that is Apache 2.0 as AGPL. You need explicit approval of every single contributor" is false.
Anyone can take an existing Apache 2.0 project and change the license to AGPL 3.0. This does not require the approval of any prior contributor. Of course people can continue using versions released under Apache 2.0 under the terms of that license, but even then "approval of every single contributor" is irrelevant. See https://www.apache.org/licenses/GPL-compatibility.html
The situation where you do need the approval of past contributors is when you want to switch between incompatible licenses. For example, if they had previously released it as AGPL 3.0 and wanted to move to Apache 2.0.
Anyone can take an existing Apache 2.0 project and change the license to AGPL 3.0. This does not require the approval of any prior contributor. Of course people can continue using versions released under Apache 2.0 under the terms of that license, but even then "approval of every single contributor" is irrelevant. See https://www.apache.org/licenses/GPL-compatibility.html
The situation where you do need the approval of past contributors is when you want to switch between incompatible licenses. For example, if they had previously released it as AGPL 3.0 and wanted to move to Apache 2.0.
(also not a lawyer)