you could use D16A6 pistons in a D16A9, however, the are not the same - the PM6 piston from the D16A6 is a dished piston and the PM7/P29 piston from the D16A9 is a domed piston - putting the D16A6 pistons in a D16A9 will lower the compression ratio to 8.25:1
this is a D16A6 piston:
this is a D16A9 piston: