Andrew's suggestion is the simplest way to get access back to all of your media. Since you have a mixture of UNC paths (\\Qnap\foo\bar\baz) and drive letter Z this will work. But you'll still have JRiver's database full of both types of references. Personally, I would want to standardize on either one or the other.
So, like you mentioned, I would want to replace JRiver's database entries for the Z: drive with references to the UNC path instead. The tool to do this job is the Rename, Move, and Copy tool.
http://wiki.jriver.com/index.php/Rename,_Move,_and_Copy_FilesHere's a high level overview of how I would do it:
A. Find all references to the Z: drive in MC. Go to a view that shows you all files. Then just type Z: into the search box at the upper right. You should now have a list of all the entries on the Z: drive that MC knows about.
B. Select some of these files, a small number to start, and run RM&C on them to change their database entries from Z: to UNC path. You'll want to use the tool in Update Database mode. This just changes MC's pointers for the files, but does NOT attempt to move or copy any files on disk. Which is exactly what you want to do. Use the Find and Replace box near the bottom of the RM&C tool to do this. Find and Replace will change whatever you tell it to in the file and path names.
Several very IMPORTANT points about this process:
1. Make a database backup before you start. That way you can restore the backup if you screw up. File > Library > Backup Library.
2. In the RM&C tool, use the PREVIEW window! It will show you exactly what it's going to do before you press OK. If it's not right, adjust it until it is right.
3. Check your work after. Make sure you can access the files after you do the Update in RM&C. If it works, move to a larger group of files.
Good luck.
Brian.