Earlier quoted context omitted.
What's your take on changing the status quo around unique task/issue ID's?
Can't say anything more than: it depends. To its essence, an ID allows interested parties to track something over time. If you add additional meanings to an ID - e.g. the parent project as Jira does, or the kind as in ServiceNow - these can become outdated over time, and deceiving. I'd say it's ok to change the "external name" as this other metadata changes over time (project name, bug, task, story, whatever...) but…
Obviously, you wouldn't want to have things migrating between projects - but if that is even concievable for an issue to do, then the division in 2 projects was probably not correct and it should perhaps have been a single project. That is: the PROJA-0001 is as immutable and unchanging as 1000001 is.
A thing changing "kind" (e.g. a bug to a feature) doesn't seem far fetched, and I'd be reluctant (Meaning I think it would be a flawed design) to use that as the public identifier prefix for a feature.