I agree that educating users about license choice and encouraging users to select licenses compatible with sharing/scientific use is a way that iNat can maximize its impact. I think it’s tough to communicate to users since licenses are so complex for the average person, and there are many different options. There are some existing threads about this (such as the current: https://forum.inaturalist.org/t/preliminary-findings-from-a-qualitative-study-on-inaturalist-users-license-choices-show-there-is-a-substantial-portion-of-inaturalist-users-who-have-all-rights-reserved-by-mistake/52742)
As a quick note of clarification, by
you mean “All Rights Reserved”, correct?
Also, kudos on contacting so many users and helping increase data-sharing! I would also note that, while the stats you have are for herps specifically, the users almost certainly changed their licenses for all their observations (not just individually for herp ones), so the amount of data newly shared is likely much larger than the numbers you report here.
I think that a message asking users with more restrictive licenses (as opposed to all users) to consider their settings could be useful. It could probably be targeted in some ways to only users who have been active on the site for more than a year (for recent users, it might seem pushy to ask so quickly?) and those with more restrictive licenses. There’s no point messaging a user who already has CC-0 or something like that. A message could have links to the user setting page and a Help/Tutorial on changing a license.