Firebase linking users across application with unique identifier

Question

I'm using firebase and my users are set up like this:

{
    "firebase-account-123": {
        "users": {
            "simplelogin:1": {
                "properties"{ "name": "john doe", "email": "[email protected]" }
                "children": {
                    "simplelogin:2":{ "name": "user 2", "email": "[email protected]" },
                }
            },
            "simplelogin:2": {
                "properties"{ "name": "user 2", "email": "[email protected]", "disabled": false }
            }
        }
}

I use simplelogin:X and the users email across many objects in my database. I know using this to identify my users across different objects is probably a bad move.

At some point I am trying to implement the ability to change emails which means that their simplelogin ID is going to have to change along with every email property that ever linked to their email.

Does anyone have any suggestions on how I can link users to one another without have to change their data if it changes across the entire platform? I'm thinking somewhere along the lines of a unique identifier for each users. I just don't know how I would implement this.