Short answer, sounds like yes you have no backup in the normal zune backup path. When you upgrade Zune it wipes out backups stored there. I am unclear on what you did with your backups when you say you downgraded to 4.7 and did a backup then did a 4.8 backup?!
When my program runs and you choose to do another backup when one exists it asks if you want to archive it. If you did this then you have a backup stored elsewhere.
This is the NORMAL Zune backup path. Paste it in to a run dialog and hit enter:
%USERPROFILE%\AppData\Local\Microsoft\Windows Phone Update
This is where a restore is performed from.
This is where my program puts a backup IF you choose to archive a backup:
%USERPROFILE%\Documents\Windows Phone 7 Zune Backup Archive
There is a backup management menu at the top with which you can browse any archived backups and put one back for restore. At this point though if you do have an archive you might want to copy it off to a safe location.
At any rate, if neither path I gave you above contains backups then I am afraid you have none to restore.
For future reference install 4.7 then copy off the zune folder in program files and look at the registry entry for it:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Zune
The Current Version value is what you want.
Now upgrade to 4.8.
To switch zune version I just make sure all Zune processes are terminated and then rename the program files\Zune path to Zune4.8 (assuming I have 4.8 installed last). I have 4.7 under program files as Zune4.7 which I just rename to Zune. Then I change the CurrentVersion registry value to reflect either the full 4.7 value or the proper full 4.8 value depending on which zune I want to backup with. When the backup tool runs it will run as whichever zune and winphone tools copy you set to be the "live" version.
Having said all that, i am not working on this anymore. Plenty of people have copied this technique and use it in their own backup tools which they are still supporting. You might want to look in to one of those.