Ella Fitzgerald wrote:
Try to put everything you need into the common folder, like /Applications.
That is terrible advice. A user should never put their personal files into the system area of the OS. A better option is to create a temporary folder in the "/Users" folder, but even then the permissions of the items will be for the original user, but it should be possible for the new user to copy them.
The new user should be able to copy the whole "Documents" folder and "Desktop" folder from the original user account since macOS will prompt for an admin password. Even though those folders have a red "X" on them it is possible to copy them using the new user account (at least it used to be possible).
If the OP just wanted to change their user name, then the OP should make sure they have good verified working backups. Only after the user confirms they have a good working backup, then and only then should the OP can change the user account name following these Apple instructions exactly:
https://support.apple.com/en-us/HT201548