BE version Backwards Compatibility

Situation:

I am trying to restore from DAT tape.    I have no idea if the backups have been working or if there is anything on the tapes.

I have no idea if the version of Backup exec that the customer has on CD that I can use to reinstall is the same version that was used to backup the data.    i.e. dont know if the software has been upgraded on the server since it was originally installed from media. 

The version of BE that I have access to is 10d on a Server 2003 r2 x32 server.

When attempting to inventory -  the operation is succussful but it indicates that there is nothing on the tapes.   most of the tapes read "unknown media label"

I dont know if the version of BE that was used to backup, if a backup was even performed correctly for that matter, is the same as what i am using to restore.

I installed BE exec 2012.    2012 indicates that the media is scratch.

My question is:    It is an assumption that BE is not forward compatible, that is to say version 10d will not read 12.5, etc.   However,  what is the version backward compatibilty?   Can 2012 Read a tape that was backed up with 10d?    Upon attempting to install 2012 over 10d it indicated that an interum version be installed (v12) and that a direct  in place upgrade  from 10d to 2012 was not possible.

My goal is to see if in fact there is anything on any of the tapes.   How i get there, is not important.     At this point, it appears that in fact the backups have not been working.

Thanks in advance.