This may or may not help...

I had the same situation occur. After doing the install - from the shipped drive to my C drive - over the existing install (how I have always done it), ALL the the RealTracks above 300 were listed as N/A.

But... I could still load them into a song in BIAB. They even appeared in the mixer with the N/A in front of the RealTrack name. But, they still regenerated - they were actually there and usable.

However... after I went to the Style Picker and did a Rebuild (a fast one), all of the N/A designations disappeared.