I joined Envato as an affiliate, and now I am an author.
The username I choose was just random, but now it’s a problem.
So I want to change my username, but want to ask if it will affect my earnings, sales stats.
I mean, will they get reset to 0 or something. Because on the account.envato.com it says
Changes to your username may have an effect on URLs to your profile, items or services.
As far as I know changing your username won’t have effects on earnings or sales count, however, all your referral links will no longer generate income for you so you’d have to do that tedious task of updating them to match your new username.
It’s better to update your affiliate links but all the information will be probably transfered to new username.
I have changed mine too, like three years ago. If you visit the old account, there’s a redirection so visitors automatically checks my new account page ( new user ID )
The system will do a redirect to your new profile rightaway.
I’ve changed mine 2 months ago for easier brand recognition. Considering the market is a little bit down --> have no idea it is affected by the change or not. 
Anyway, in your case, if you are new authors and have no old (returning) customers thought, I see no problems with this.