I'm pretty sure you do not get a pro-rated refund from AMEX if your card is closed due to no longer having an MS account. You could wait until your engagement bonus posts and then immediately cancel the card as from what I've seen on my account you'll still be within the 30-day window. Or if that doesn't work out--the bonus doesn't post--just cancel the card right before the 30 day window closes and you may get the engagement bonus anyway. Your other option is to keep some kind of account open at MS that has minimal fees, like th Pathways accounts.