Just the creation of a device will not touch the media at all. NW will only delete the contents if you (re-)label the volume. NEVER do this if you want to scan/recover later. Also make sure that you set the retention period for the client(s) long enough. Otherwise NW will scan the save set but later see that it has passed the retention date and delete it during its internal cleanup processes. So you better set the media write protected or the device to read only. To recover backups from an obsolete client, you can also add the hostname as a fake entry in the hosts file. Then just run a 'directed recovery' to restore the files to the new machine.    
I had a situation were our Networker 8.2.3.0 lost it's configuration due to IP/domain name changes and required me to re-configure it from the start.  During that period of time, when I added one of the two adv_file devices that was created in the previous configuration, it deleted all of of the data.  I figured we were still good since all of our tapes should still be good.  I have started doing a scanner -i -v \\.\tapeX on most of our tapes and I can see that one of our required backups resides in the other adv_file device, the one that was not deleted. How can I add this adv_file back to Networker and NOT have it delete all the files again?   If I can't do that, how can I do a restore of a system that is backed up to tape but no longer matches any currently configured client names?  I can see the server I need in the index by doing mminfo, and see that there is a full backup located on both tape and this other adv_file location.  When I attempt to do a restore, this server name never shows up and attempting to do mminfo -c fails to find the records returned from mminfo that listed the "old client name".   The "old client name" is listed in networker as one of it's aliases. 
adv_file after server rename