Yes you can change it to your domain later. 1) You will need to purchase your own domain. 2) You will need to migrate the contents again, since each wordpress installation is tied to the domain/subdomain.

For the subdomain, it will always be the same as your steemit username. This is for proving your identity (we don't want some spammer to request for dan.vornix.blog :) ), and to protect the availability of the name just in case the steemian decides to use our service.

Don't worry about copyright. It doesn't apply to us.