If you have already configured ssrs on the new instance then it is more challenging. The best thing to do is to install but don't configure ssrs on the new server. Backup and restore your reportserver and repoetservertempdb databases from previous server over to new one. Then manually configure ssrs from the ssrs configuration manager where you will attach the ssrs databases to your new ssrs instance. Once you do this and start running your reports then they will update and everything should be fine.