Do you lose your upgrades?

I've always wondered about this and now I suspect it will become a reality.
If you are on a booking that has been upgraded with miles, and your flight is merged with another because of low loads (or cancelled for some other reason), are you guaranteed your upgrade or does it depend on reward availability on the new flight?
I assume and hope it is the former - but is that right?
Thanks for all help
If you are on a booking that has been upgraded with miles, and your flight is merged with another because of low loads (or cancelled for some other reason), are you guaranteed your upgrade or does it depend on reward availability on the new flight?
I assume and hope it is the former - but is that right?
Thanks for all help
