Reassign users from old user role to new user role with anonymous Apex

ReassignUserRoleJob.java

private class ReassignUserRoleJob {
    
    private String oldUserRoleName;
    private String newUserRoleName;
    
    public ReassignUserRoleJob(String oldUserRoleName, String newUserRoleName) {
        this.oldUserRoleName = oldUserRoleName;
        this.newUserRoleName = newUserRoleName;
    }
    
    public void execute() {
        List<User> affectedUsers = [
            SELECT Id
            FROM User
            WHERE UserRoleId = :this.getUserRoleId(this.oldUserRoleName)
        ];
        
        Id newUserRoleId = this.getUserRoleId(this.newUserRoleName);
        
        for (User eachUser : affectedUsers) {
            eachUser.UserRoleId = newUserRoleId;
        }
        
        update affectedUsers;
    }
    
    public Id getUserRoleId(String name) {
        return name == null ? null : [
            SELECT Id
            FROM UserRole
            WHERE Name = :name
        ].Id;
    }
}

String oldUserRoleName = 'Old UserRole Name';
String newUserRoleName = 'New UserRole Name';

new ReassignUserRoleJob(oldUserRoleName, newUserRoleName).execute();