Well that would be up to the league owner... I know I personally HATED yahoo. ESPN had bumps their first year, but since I have LOVED the customizing options they allow, as CBS and Yahoo have been "our way or the highway" for free leagues.
Dunno just my $.02
I would guess that if we went Yahoo we would lose my wife, who wouldn't be apt to sign up in another league as she has only done ESPN.
GE-Raven