Development of Creative Commons licenses

["Erik Moeller" <eloquence AT gmail.com>]

Dear all,

I run a project called FlickrLickr (
http://commons.wikimedia.org/wiki/User:FlickrLickr ), a collaborative
vetting process to choose freely licensed photos from Flickr which are
suitable for the Wikimedia Commons, the free media archive used by
Wikipedia and its sister projects.

My script has populated a database with metadata about Flickr photos.
Users then choose, based on the information in my database, which
photos should be uploaded to Commons.

Unfortunately, Flickr makes it trivial to change the licensing for all
photos by a user retroactively. So it is quite common that images
which were labeled as free by the time my script spidered them, or
even by the time they were uploaded, are relicensed later to a more
restrictive license.

My question:

What are our legal obligations? Do we have to retroactively delete
these photos, or can we state that, since they were once flagged as
being e.g. CC-BY, they can be assumed to always be CC-BY, no matter
what Flickr says?

Thanks,
Erik