Is there any way you can check this in the "Fare Conditions" just prior to booking. Wouldn't be of much use if one was informed of this after booking.
I tried a dummy BOM-JFK saver and found this in the fare conditions. Buy my guess is that this clause if for a 'paid upgrade' and not an upgrade with miles?
"AT ANYTIME WITHIN THE TICKET VALIDITY FARES CAN
BE UPGRADED TO ANY FARE IN A HIGHER CABIN"