BLDC Motor Performance at high altitude?
I've sometimes discussed motors for space applications with high-end motor vendors.  While I can't say this is conclusive, different torque curves for ground testing vs space were never brought up by the vendor.

However, one thing that did come up is lubrication.  As mentioned by several comments and answers, your motors are at risk of running far hotter than on the ground due to reduced convection cooling.  However, you also have a risk at the cold end. Will your motor ever be powered off for a significant period of time?  (I'm guessing this is a balloon payload?)  If powered off, the motor could cool until the lubricant thickens too much, causing the motor to bind when you turn it on.  Your motor should list its minimum operating temperature.