Thanks.
This is now my official decision:
Based on the complexity of those terms, I won't do any tools to handle the transfer. Also, this is a free site and serving potentially millions of their embedded images (as a "cache") is not possible because of bandwidth costs.
We have an image upload feature here, and a post edit feature. That should be enough if anyone wants to salvage some of their posts here. If you re-upload images that are in your gallery you will need to re-add them after upload, same goes to EXIF/metadata, unless you upload images with embedded EXIF.
If I were you, I would first wait to see what images really get deleted, if any.
It is a mess, but as said, this is a free service and there a limits to efforts I can/will do to remedy some commercial megasite's greedy change of service. Many Flickr images won't load after that change, but a lot will do - I'm sure not all POTN members belong to top 3% of Flickr users.
What I can do, is make a info tool that lists your Flickr images posted, and if they actually load or not.
There is also already an internal tool that lets us check that all member's gallery images actually load, and you can remove images from gallery there. I can make that tool public later.