Requests for User Generated Panoramas using the old pano value in Maps API JS (StreetViewPanoramaOptions) or using iframe embeds will still work, but if you try to find a panorama using a position param you will get the new id back. There is no need to map the old and new pano ids as both will remain valid.
Please note, that because we cannot guarantee the format of pano ids will stay the same, it is recommended not to depend on pano id parsing and/or verification logic.
Here is an example of old and new user generated pano ids:
If you notice any issues please report them via our issue tracker (https://issuetracker.google.com/issues/new?component=188853&template=788207)
Google Maps APIs Team