bri@screwtape essentially, as long as there's a single copy of each out in the wild, you have two otherwise identical products, each with separate licenses. cc sorta kinda outlines what goes into upgrading at https://wiki.creativecommons.org/wiki/4.0_upgrade_guidelines#Logistics_of_applying_4.0, but there isn't much to it because again you're just sort of... doing a new one.
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