The format of the issue key is so fundamentally baked into JIRA, that if you changed this somehow, it would break a plethora of things.
Why don't you use a textual custom field for your own key, and force the format using a regex? (There are apps in the Atlassian Marketplace that offer this custom field type.)
Hi all! Lauren here from the Enterprise Cloud marketing team. I have some exciting Cloud updates from #AtlassianSummit that I wanted to share. We want everyone in your organization to be ab...
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