You can change both your WordPress.com account username (the name you use to login) and your display name (the name that is seen on your posts and comments).
Once you change your username you will not be able to change it back and the old name will not be available for you or anyone to use. If you just want to show your name differently please only change your Display Name.
So it sounds like you've already changed it once. But you can still change that display name - and that's all anyone will see! Here's the detailed information on how to do that (with a handy video!)
Since you've registered the old address, your only option to get it back is log in as the old username and transfer it to yourself:
But you could bypass this altogether and register a new domain for the blog, so if your blog URL is therecesshionista.wordpress.com, you could purchase the domain Giselakas.com by going to your Dashboard > Store > Domains, and it will automatically map to your blog, so all users will see is "giselakas.com" instead of "therecesshionista.wordpress.com" OR "giselakas.wordpress.com."
Let me know if you have further questions or you get hung up anywhere!