If anyone started using OpsGenie post acquisition they should have KNOWN atlassian would change something, assuming it would remain status quo would have been naive.

For us it was a seamless migration for us to Jira Service Management from OpsGenie. There were many emails, and many in app banner notifications for MONTHS. All our rules transferred over. I only had to setup our notification integrations so all links into email/slack goto Jira Service Mgmt instead of OpsGeneie. Our OpsGeneie instance is currently read only.