This is the official answer from help page,
hoping that they will change in future

What happens if my email or phone number changes?

Currently there is no way to change the email or phone number that is linked to your account. Though once your account is created, you can continue to use it even if the email or phone number that is linked to the account has changed.

https://steemit.com/faq.html#What_happens_if_my_email_or_phone_number_changes