There’s an odd aspect to this…
PersonA is present in CityStrides today & completes CityA tomorrow. PersonB joins CityStrides next week, but they completed CityA last month and 
didn’t know about CityStrides while they were doing it.
From what I’ve learned in building CityStrides so far, people do not react positively to changes in “truth” (with good reason). So, I couldn’t really “move” the badge to PersonB because then I’d have to handle support requests from PersonA (multiplied by the 60k+ active cities, that’s a new full time job 
).
Though… If I can clearly document that it is not a permanent badge, then that could work.