Thank you for your post.
Could you let me know which version of OS you were using and which one you went to?
You shouldn’t have to reinstall the app or anything, when the OS is upgraded. However, if you do delete the app then it also deletes any maps and map features. Therefore, there isn’t anything you can do to recover these.
In the future, you can prevent this by export the map features: https://support.avenzamaps.com/hc/en-us/articles/360035481091-Exporting-map-features-from-a-map
If you have any questions don’t hesitate to let me know.