How to change screensaver user name and/or iterm2 top bar name on mac
If you want to properly change your account username, it is advisable to create a new account to ensure that absolutely nothing goes wrong and everything continues working as expected. However, it is very likely that someone may want to change their macOS user account name and/or home folder name, and Apple conveniently has a support article this. 

I'm not sure which of these steps you may not have taken, so take a look at the support article and find out what you may have missed. As with any system modifications, I would suggest you to create a working backup with you can use in case anything goes wrong.