Symantec Management Platform (Notification Server)

I have a very simple task that we run across the school district to shut down PCs at night.  (you know, green and everything)  We're using a simple script with psshutdown.  Now, when I select the devices, I can use a quick add and then use a filter based on the first few characters of the PC name since all the PCs at a given school start with the same characters.  I have also selected "Add" then "Computers or Devices" and pick the OU where the computers are.  Now once I have the group of devices set, no matter how I picked them, it doesn't seem like I can modifiy it.  If I select "add" and then"Target" then setup a filter that excludes a specific OU at the school, it will take the filter, but when I go back and look at the details of the task, the PCs that were filtered were still there and they still get shut down at night.  It's not a matter of refresh.  (I also saved the "Target" filter so I could use it again)  The only way I could remove them was to delete the next runnng of the task and then create a new schedule using the filter I saved.  Is there no a way to modifiy the set of PCs without creating a new schedule?

Thanks,
Tim

Previous discussion, and I believe what you are seeing is something similar. In my past experience, the only reliable way to do what you want is to confirm what you have already found out. Delete the schedule and start over. Sorry that I can't be of additional help.

I actually read that thread, but being pretty green, I wasn't sure if I was experiencing the same issue.  Thanks for the response.  It was helpful. 
Tim