Stable Story IDs?

Would it be possible for the human-visible story IDs (at least the numeric parts) to not change when moved from one theme to another? At least as an option (per backlog)?

Our process assumes that once an ID is assigned to a story, that story has that ID forever (so we can refer to stories from other documents). I found it very surprising that moving a story from one theme to another changes not only its ID prefix, but also the number. Obviously this could be worked around manually by editing the number as soon as the story lands in the new theme, but... what a pain! (Plus, someone will undoubtedly forget.)

Ideally I'd also like to see the prefix not depend on the theme either -- but could probably live without that, as long as there's a way for the numbers to be stable