If I were you I'd open the store on your current selling account and call it by the new name you want to use. In 30 days time change the name on the new account, then 30 days later change your current selling account's username to the new username.

The 60 day wait will give your regular buyers a chance to get used to your new store name and will know who you are when they eventually see your new username in their feedback. That way if they want to find you again after several months they're more likely to be able to do so.