Okay, found the answer 8-)
So yes, I need to rebuild the custom app (and therefore resubmit to Apple) to upgrade a folio made with 1.5.0.
Please Adobe guys correct me if I'm wrong.
Another question: using the Folio Producer web pages online, is there a way to know which version has been used to produce a folio?
That's correct. If you install the newest Folio Producer tools, you need to use Viewer Builder to update your custom viewer app. By the way, that's why you can update the Folio Builder panel separately - so you can stay on an older set of tools until you're ready to update the viewer.
Whenever a new Viewer Builder or Folio Builder panel is available, and update it required. Updating the Folio Producer tools (authoring tools) is optional.