There are two unknowns:
a) whether 'upgrade at OLCI' involves a substantially discounted quantity of miles, or the same as would be required for a conventional upgrade at the airport. I would be surprised if there is a substantial discount - this has been advertised to Golds, and I think the reason Golds do not receive the cheap 'cash upgrade' emails is that too many discounted upgrades would dilute J and F revenue on regularly oversold routes.
b) whether this will be available to Blues - essentially it is an upgrade at check-in time, which was previously reserved for Silver and Gold.
I will also be interested to see what inventory is used for mileage upgrades at T-24 (or earlier, if checking in for a connecting flight also). Upgrading with miles at the airport, I've managed to get the last F seat on a flight (when there was no Z available). I don't think it's in EK's interest to allow the last F or J seats be filled by upgrades 36 hours before the flight, when there is a chance of selling full fare F or J tickets.