Turkish Airlines Miles&Smiles - *A Upgrade on TK?

Can someone please direct me to a thread that discusses the use of *A upgrades on TK? My search results through the forums have been unsuccessful.

The question is, does TK permit a *A upgrade from Y or B class into any open business class seat, or must there be I class available? (I have received conflicting information from Aeroplan and TK.)

Thanks very much.

You can actually upgrade to D,C or J due to availability.