How might I go about doing that?
Go to “Account Settings”, and then “Content and Display” (top left).
Scroll down to “Licensing”, and select your new default image license.
Check the little box below to apply it to all your previous observation images and hit “Save settings”.
Here is a short (1:44) video made by @andrawaag (and narrated by @siobhanleachman) that demonstrates it nicely.