When I got my 8320....October-ish.... I upgraded the YM that came with it to the one I got from the BB mobile site... mobile.blackberry.com. I had no problems.
I did it again over the weekend. I was running OS .174 I think....I upgraded it to .180 form the tmobile site. It reset it to the old one, but I again downloaded and upgraded to the no one with no problems. If I go to Settings - Advanced Options - Applications it shows I have Yahoo! Messenger 2.0.38
i use Yahoo all the time to talk to one of my daughters and some friends. Works fine for me.
Sorry I can't be of more help... but it did install and works fine for me. Maybe someone else can take that and give suggestions.