The SDK's simple-prefs module stores its settings in a preference branch which is tied to the add-on's ID. There shouldn't be any risk in automatically migrating these preference values to storage.local keys the first time an add-on is upgraded to a WebExtension with the same ID. This technically applies to other legacy add-ons as well, so long as they store their preferences under "extensions.<add-on-id>.", but I don't think very many of them do. However, authors of legacy add-ons that want to take advantage of this feature could fairly easily rename their preferences several versions before they finally migrate to a full WebExtension.

Note that we are likely going to change the implementation of storge.local (which currently uses a json file) to use a Kinto collection which is what storage.sync will use. Please keep that in mind when designing this migration routine.

Going to assume that at this stage this is not happening. For migration we have the Embedded WebExtension and have been recommending that to add-on authors. https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Embedded_WebExtensions