Hi!
I can tell you first hand the difference between Diane 35 and Yasmin and "regular" BCP plus Spironolactone (I have been on them ALL for extended periods of time). If you are switching to Yasmin from Diane, you WILL need additional spironolactone. Trust me

All the great anti-androgenic benefits of Diane will be lost if your doctor does not supplement the Yasmin with additional spiro. Yasmin just isn't strong enough - don't forget that Yasmin is prescibed for "normal" women who don't have PCOS.
I had a doctor (who wasn't an endocrinologist) switch me from Diane to Yasmin without additional spiro, and that was the biggest mistake of my life! Bye-bye hair on my head, hello hair *everywhere* else (arms, belly, face). It was like all my cosmetic pcos symptoms returned with a vengeance. The amount of anti-androgen in Yasmin is MUCH less than the amount in Diane 35, even though they are different drugs.
The Cyproterone Acetate in Diane 35 is a much stronger (and better) anti-androgen than the Drospirinone (which is related to sprironolactone) in Yasmin. The amount of anti-androgen in Diane is over 4 times stronger than the one in Yasmin. Basically, the amount of Drospirinone in Yasmin is equivalent to 25 mg of Spironolactone. Most PCOSers start out with 100 mgs of Spiro in addition to their regular BCP. You would need an additional 75 mgs of Spiro taken with Yasmin to even come close to Diane.
When I was on Ortho tri-cyclen with 100 mg of Spiro, it controlled my acne and hair problems, but not as well as Diane 35. The Yasmin alone made my symptoms worse. Since my Yasmin ordeal, my new endo (YAY!) has switched me to Ortho Tri-Cyclen, 100 mg of Spiro, and 1500 mg of Metformin XR. We're pretty confident that the Met and Spiro will do their jobs regarding the hair problems, but it will take at least 6 months to see results...I lost a *significant* amount of hair on Yasmin, and I'm just really hoping it will grow back, I'm sick of wearing hats!
One of the side effects of spiro and the drospirinone is breast growth/swelling - I don't mind, because mine were never that big to begin with, so I welcome the side-effect
But, everyone is different, so it's up to you and your doctor to discuss your protocol - I just hate for people to think that Diane 35 and Yasmin are interchangeable; alone they're not!
Btw, my endo suggests that if I move back to europe that I go back on Diane. It is really the best pill for PCOS; it such a shame that it's not legal in the US.
Good luck!