You could uncheck sharing the items with the "old" group, and check for sharing item with the new group.
Share items—ArcGIS Online Help | ArcGIS
You can change your First Name and Last Name without changing the UserName.
Else, if there is room to create another user account, you could create one with the desired username, grant it Administrator privileges, and move the items created by the administrator. Then, you could delete the older User Account.
Think Location