After the purchase, you get a Minecraft Premium account (Java Edition)
This is a fully licensed Mojang account with Minecraft Premium (Java Edition) with mail access. You can change absolutely all data.
Each account has security security questions. We installed them ourselves, so that it would be possible without having to go to the mail that is tied to the account to change the mail to ours through the Mojang website
In any case, the secrets can be reset by mail, even if you do not know the answer to them. This is the advantage of a mail account

A new mail has been registered for each Minecraft Premium account, which you can leave or
bind your IP after the expiration of the period of time blocking by the mojang security system of 30 days or less, you cannot change mail more often, the mojang security system also temporarily blocks it, as when changing the IP address of a PC. We recommend that you leave this mail, first secure it as much as possible (bind your phone, change the secret, passwords, etc.).
Questions "when is it already possible to change mail?" do not ask, each individually, does not depend on the seller. You are provided with clean, only registered mail, use.
The mail you want to change to should not be registered on the site https://mojang.com/, as well as https://minecraft.net/

Change nickname, password, security questions and mail here https://account.mojang.com/login
Skin change here https://my.minecraft.net/en-us/login/
Download the official launcher to play https://www.minecraft.net/ru-ru/download/