It is not possible to migrate a character to another account by yourself. Also note that it is against the rules to create a second Master Account, even if you don't use the other one.
What you might be able to try is create a support ticket here: https://redcountyrp.com/tickets , and ask for your Master Account name to be changed. Include an email address in this support ticket so you can be contacted with your new account information.
This is not a guarantee that your request will be accepted; issues like this are mostly not prioritized because few people but yourself know about your Master Account username.
If you have any other questions, feel welcome to ask them.