Seattlenerd - They automatically upgraded me to a new Signature card, and told me that I can still charge my Platinum card. I was told the Signature card is being mailed to me.

1) Does this mean that since they automatically changed my card to Signature that I won't get the 20k miles from Alaska Airlines?
2) Should I not activate or cancel my Signature card? Will doing this allow me to apply for a new Signature card and get the 20k miles?
3) I didn't really understand your post on how the Signature card affects negatively on my credit rating - can you elaborate?