Definition of 

Downgrade from Grace Period

What's 
Downgrade from Grace Period
: it is one of the possible values for
Event
. It means:
The App Store successfully completed the transaction within a 6 or 16 Billing Grace Period window, and switched from a subscription in a higher level to a subscription in a lower level.

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Trends: Subscriptions Events

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App Store Connect

The (only) data hub made for apps