I want the username on my main account to be changed to the username on this account. Basically, if I were to purchase 1k robux on this account and on my main, what would happen if I changed the username on this account to something else and then changed the username on my main account to this account’s username? Would this work?
When you change your username it’s still being held by the account, so if you tried to change it and then create a new account it wouldn’t work. Your previous usernames are “reserved” for you in case you want to change it back later. Hope it helps!
So I can’t swap usernames with different accounts?
No, you can’t. For reference,
( Changing Your Username – Roblox Support)
Once you set a username you own that username and no one else can get it, even if you change it later.