I'll mention right up front that most of the more than 10,000 photos I've published on Flickr were, and continue to be, under the Creative Commons Attribution Share-Alike licence. Most of the people who have used my photos have done so quietly, without any fuss. Now and again, someone asks politely if they can use a photo even though it is clearly marked as to which licence applies, and my Flickr Profile also makes this clear and goes so far as to say how I prefer to be attributed. If I think that anyone wanting to use my photos might have problems with the Share-Alike part (e.g. commercial enterprises who want to keep their own licences on the content) I just generally quietly change the licence to Attribution only and let them get on with it. I may be unusual in this way as my intent is to provide a service to the community - and that's what I see CC as being all about. And as a result, many hundreds of web pages feature my photos. And this probably saves them a dollar or two. It looks like a win-win situation from here.
I do, however, reserve the right to change the licence on any of my photos, no matter when they were first published. I'm trying to build up a small fine art portfolio and intend to continue changing some photos to All Rights Reserved. And I've stated as such in my Flickr Profile. But that in no way affects the rights of anyone who has used any of my photos beforehand. As far as I'm concerned, the date of publication should settle the issue of who has which rights. If someone used my photo under CC BY-SA on the 31st of March, say, and I change the licence to ARR on the 1st of April, then that person's rights don't change as regards their use of the photo up to and including 31st March. They don't have to take the photo down, and I won't charge them for its use. If they reuse the photo after that then it's likely to be because they don't know about the licence change. If I find out they've reused it, I'll let them know about the licence but won't insist that they find another photo. After all, I might not know they've reused it until months afterwards. So I'll always err on the generous side. So that I can continue providing a service. At some point in the near future, I'll convert most of my photos to CC BY to make this easier.
Having said all this, I know that some other photographers are far less easy-going, even to the point of being litigious. My opinion is that the Creative Commons (i.e. whoever 'officially' represents it) needs to look at the wording on the CC website
and especially any legalese therein, and rewrite it to include information and guidelines as to what should happen when a contributor intends to change their licence. If, as I believe is the case legally, the date of publication is primary in deciding who has what rights, then the website should make this clear to both contributors and users of creative work. Users should be clear that, if the licence on a work changes, their use up to that date is lawful and cannot be challenged; and that reuse of the same work after the date of change of the licence comes under the provisions of the new licence exclusively. Likewise, contributors should also abide by this and not attempt to charge fees in retrospect for lawful use of a work prior to the date of change of the licence.
For me, the spirit of the CC movement is most important, but the letter needs to be nailed down a lot tighter than it currently is.