Modify a Google App Engine entity id?
The entity ID forms part of the primary key for the entity, so there's no way to change it. Changing it is identical to creating a new entity with the new key and deleting the old one - which is one thing you can do, if you want.
A better solution would be to create a PhoneNumber kind that provides a reference to the associated User, allowing you to do lookups with get operations, but not requiring every user to have exactly one phone number.
I'm using Google App Engine NDB. Sometimes I will want to get all users with a phone number in a specified list. Using queries is extremely expensive for this, so I thought I'll just make the id value of the User entity the phone number of the user so I can fetch directly by ids.
The problem is that the phone number field is optional, so initially a User entity is created without a phone number, and thus no value for id. So it would be created user = User() as opposed to user = User(id = phone_number). 
So when a user at a later point decides to add a phone number to his account, is there anyway to modify that User entity's id value to the new phone number?