We did just that in October and took a bus from the Poly to Downtown Disney then changed to a bus to BC/YC. Our ownly complaint was we waited for a while at the Poly for a bus to come and they really don't have an area with benches like most other resorts do.
If the parks are closed, the only option using Disney transportation is to do as paras4ri did - take the bus to DD/PI and then transfer to the bus for the YC. It can take an hour or more depending on how long you have to wait for the two buses.
IMHO, the "best" way is to take a cab. That will cost from $10-$15 including a tip for the driver.