If you have added suitable dither yourself, just convert the bit-depth with all Soundforge's dither options turned off.

Did you select 16 bit in the L2?

I have never used dither in sound forge. But when I used my L1 ultramaximizer to dither in Bias Peak, I had the same problem. I set the L1 to dither to 16 bit and after it rendered is still showed it as a 24 bit file. What I had to do is change both Bias Peak and the L1 to 16 bit before I rendered.

-Danger-Rus

The plugin can't change the bit depth... that's up to the app to manage. Tyhe plugin simply applies dither of the appropriate level in order to prepare a file for bit rate reduction.