Every WordPress.COM blog has a sole owner. It is the person with the WordPress.COM username account who registered the blog under that username account and a single associated email address, and who is the original Admin of the blogs registered under that username account.
The owner of the site can transfer the site (and/or upgrades) to another account by adding another username account as an Admin.
Transferring a Blog to Another WordPress.com User or Account https://en.support.wordpress.com/transferring-a-site-to-another-wordpress-com-account/
Transferring upgrades https://en.support.wordpress.com/my-upgrades/#transferring-upgrades
Change Credit Card Details
https://en.support.wordpress.com/change-credit-card-details/