So I called.
As of 3/3/12 -- maybe +/- a day or two, I'm half asleep, sorry. This card will be transferring over to a new, yet to be developed (or at least published) card under the United Brand. It is definitely NOT going to become the Explorer Card.
Between now and then, the benefits will be the same as they have always been (up to 8 people's bag fee being the main one).
One CAN combine this card with the Explorer card (without taking the hit of closing an account), however, if the Platnum Card has a balance, one cannot combine cards until the balance is $0. -- And one cannot do a Chase-Chase balance transfer.
Even if we could transfer balances from one Chase account to another, and this could because my Explorer card has been open less than a week, I currently have no ability to transfer balances onto the Explorer Card.
For the minority of us who have this older discontinued card, I hope this helps. I'm actually surprised I didn't get solicited to "upgrade" at my account renewal early this year.