Upgrading into international business class on a refundable economy ticket
I booked 2 B fare class (refundable economy) tickets for JFK-BRU in November. It cost about $3500/person. I won't know for sure whether I will be traveling this itinerary for a few months. (As of today, a non-refundible ticket in business Z is $5135, and refundable business D is $8195.)
I do know for sure that if I do end up traveling, I would like to be in business class, so my plan is when I decide definitively, either to cancel the B booking and purchase a non-refundable business class fare (on DL or a competitor), or use miles/cash to pay for the upgrade. (I might have Delta silver status by then, but probably not). But it occurred to me that perhaps I could call Delta now and ask to use miles to upgrade and have those miles redeposited if I cancel the itinerary. Is that possible?
What is FlyerTalk's strategic advice here to maximize the likelihood of business class at a reasonable expense? (Say, less than the difference between Z and B.)