Allow transfer of primary admin role in Proton Family
There are multiple reasons why a primary admin may wish to transfer that role to another email address. For example, the family's threat model has changed since initial configuration, and it has become necessary to segregate ownership of billing, file ownership, and passwords to a standalone admin account accessible only from home and not during travel. With this setup, the primary admin account could not be compromised during travel.

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Juan commented
This is critically important. Many Family Plans were created spontaneously from one account which is not necessarily a privileged or "primary" member of the family. One of the primary reasons people subscribe to a Family Plan is to create one or more account shared between members of the family -- for example, an account owned by both a husband and a wife, who also have their own private accounts. Once the usage process is mature, the husband and wife would naturally want to shift billing responsibilities to a shared Proton account, rather than having it locked to the account which initially created the Family Plan. What if the husband created the Family Plan and is the "primary" admin, but he dies? The wife would have no way to pay the bill!
Please, PLEASE add the ability to transfer the "primary" status to another member of a Family (or even "Duo") plan. It isn't a niche issue, it's probably a central issue in all Family Plans.