Why only in first install of application the registrationManager is send in Marketing Cloud SDK Android?

Question

Hi in the past of my app we were using phone number as ContactKey but in February this changes to a personal UUID generated and stored in database.

This is an example like with the phone number

MarketingCloudSdk.requestSdk { sdk ->
        val registrationManager = sdk.registrationManager

        registrationManager.edit().run {
            setContactKey(phoneNumber)
            setAttribute("channel", "app")
            commit()
        }
    }

And this is the new code

MarketingCloudSdk.requestSdk { sdk ->
                val registrationManager = sdk.registrationManager

                registrationManager.edit().run {
                    setContactKey(accountId)
                    setAttribute("channel", "app")
                    commit()
                }
            }

This code executes in application open (when the user is already login) or when the user is just going to login, but the event for registration is only send when the user uninstall the application and reinstall and login or in the first install, and for all the users whose doesn't do this is not updating the ContactKey and they still have the phoneNumber as ContactKey, but my question is Why only in first install of application the registration is send?

I don't want the users have to uninstall and reinstall to update the ContactKey so I don't know if there is other way to do this or this is how the SDK working