Here's a data point:
Booked EWR-LAX Blue for $204/pp and was able to apply 1 mint certificate for the upgrade.
Cancelled and rebooked at $162/pp but it was now 2 mint certs for the upgrade. The mint price was $1116.
The mint price dropped to $852 and it went back to 1 mint cert for the upgrade.
When the mint price dropped it went from 2 certs back to 1. None of the mint seats were assigned at any point.