Does the old media set still exist?
If so, just add your new tapes, inventory them, and run the jobs.
The job doesn't care about the tapes. It only cares about the media set it's supposed to use. When the job runs and 'goes' to the media set you designated, it will either find a usable tape (that you have placed in the media set) or it will go to the Scratch Media set and move a tape to the proper media set.
How to move a tape to a media set?
When you add new tapes and inventory them, they will show up as either Scratch Media or Imported Media (go to the Media Tab). You can select Scratch Media and any tapes in that set will show up in the details pane. To move one or more, select the tape(s) you want to move and drag it/them to the appropriate media set.
That's all there is to it.
-SQLBill