An Allegro BRD file is a binary file. However, you can look at it with Wordpad text editor. Somewhere near the top of the file will be an entry that looks like "allv15-76/13/allv15-72/27/.............netv15-712/5/allv15". If it says 15-7xx, then it's version 15.7. If it says 16-xx it's version 16.x.
There's no way to save a version 16 file back to 15.2, but a version 15.7 file can be saved from Allegro 15.7 to a format compatible with the earlier version 15 or 15.2 format.
I'd be willing to take a look at your file for you, and downconvert it if I can. If you want me to take a shot, zip or rar it and PM it to me.