I'd like to test CANCEL events...
My understanding is that when users switch between two items within the same group (down/up grades) - the old one is CANCELed and a new one PURCHASEd.
How would I simulate that in a sandbox?
The iTunes SUBSCRIPTION manager doesn't seem to be available on the device when I'm logged into a sandbox account. I'd also like to change subscription status from there also.
You cannot manage subscriptions in the sandbox, but there is nothing special about the upgrade/downgrade process. You just check the in_app field of the receipt as per usual.
For a downgrade, you will receive a purchase event/updated receipt containing the new, lower, subscription product at the end of the current period. If you validate the receipt then you will also see the new product in the auto_renew_product_id that indicates the product that will renew at the end of the current period
For an upgrade, you will receive a purchase event and updated receipt immediately.