how to open allegro 15.7 libraries in 15.2 allegro

how to open allegro 15.7 board file in 15.2 allegro

Hi all

I have some board file that created using allegro 15.7......

When i try to open it with allegro 15.2....it gave an error message....


Is there any procedure to convert board file from allegro17.7 to allegro15.2...

Thanks