Mobile client version skew

At 15:00 you deploy backend v77, which changes a JSON API response: the field 'is_premium' (boolean) is replaced with 'subscription_tier' (string). Web clients update instantly and are fine. But over the next 6 hours, crash reports from the iOS app climb steadily, all on a null-pointer reading 'is_premium', and they're disproportionately from older app versions. App Store review for the new client build is still pending. Triage and mitigate.