Originally Posted by sdsearch

All airline miles stay in the airline account once they've posted there (unless you let them expire). The only airline miles you might lose by closing (or donwgrading) an airline miles card is those miles that are due to you based on purchases made right before closing but which haven't posted to the airline yet.

This is true for all airline programs and all airline cards.

(Things works quite differently for points programs maintained by the credit cards themselves, such as Barclay Arrival points, Citi TYP points, Amex MR points, Chase UR points, etc. But with airline cards, the card and the airline program are totally separate.)

Now, while you had the card, using the card periodically kept the miles from expiring. Once you dump the card, make sure you know some other way how to keep the miles from expiring.