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.)

Upgrading into international business class on a refundable economy ticket
 

Given that you have a B fare, if your dates are solidified and there is upgrade space, change to an M fare and upgrade with miles. If you care for the 1.5x MQM bonus, then stick with the B fare and upgrade with miles.

Can we assume that these are all RT fares?

IIRC if you upgrade with miles and then cancel the ticket, there will be a $150 fee to have the miles returned. It's still less than the miles are worth.

FULL Y class fares require fewer miles to upgrade and might not cost much more than B.

Also, are you sure that your B fare is FULLY refundable with NO FEES (as well as fully changeable with no fees)? Some B fares are not and DL sometimes uses the word refundable to mean (partially) refundable for a fee.

Finally, you seem to be assuming that upgrade inventory is available on your flights. Do not assume, even if the seat map is empty.

These are all round-trip fares. Now that I look at delta.com again I see where I could have searched for Y class - rookie mistake! Y is $5017 for the same itinerary though. Big jump.

At least one of my main concerns here is about timing - should I call and try to upgrade now, or wait until I'm sure I will be flying. $150 (per person?) seems like a reasonable price for that option.

How does one check for upgrade availability (the seat map does seem mostly empty) - just call Delta?

I was able to pick my preferred Comfort+ seat so if I am stuck in economy it's not the end of the world. Just a very strong preference for business.

Yes, you need to call Delta for a mileage upgrade. It should be 25,000 miles per person each way.
This is a good use of miles if you can find the upgrade. But beware, the fees could add up if you need to make changes depending on the exact rule of your ticket. When you call to ask about the upgrade, also ask about the exact fare rules.
If your specific flight does not have upgrades, you could ask for alternative routing that has upgrade inventory.

Also, by the way, if you were to switch to Y, it would be 15,000 miles instead of 25K. Not worth the cost.

Do you have status with DL? If not, remember that E+/EC seat fees won't be refunded if you upgrade.

OP has no status, is only FO when he flies. He'll be stuck eating the C+ fees

Didn't they used to be free with Y/B fares?

I believe only preferred seats were free. Not C+

I wasn't charged a fee when I selected the C+ seats through the Delta app.

Upgrading into international business class on a refundable economy ticket
 

I stand corrected. Y, B, M fares get free C+ and preferred

Did you see this written someplace? After the last comment and I went and looked at the website, but only saw this for preferred.

Upgrading into international business class on a refundable economy ticket
 

Don't see it written anywhere, but just did a dummy booking when logged out and it's allowing me to select C+ seats for free with my SM# not in there

What fare class?

Y on JFK-BOS