Questions:
1. To the extent reprocessing is necessary, will that be done automatically (e.g. as it is for photos originally uploaded at full resolution, and now being displayed at 5K)?
2. Do all mobile OSs provide the color profile management necessary for the app to properly display photos? I know this was a challenge in the past.
3. Will there be any indication a photo is outside of sRGB, aside from EXIF?
P.S. - I'm sure you're aware, but there's a (closed) group of alpha/beta users (https://www.flickr.com/groups/flickrbeta/) always eager to help provide feedback.
2. The coverage across modern OSes, browsers, and devices is incredibly high, and increasing daily, or we wouldn't be finally pulling the trigger. (I've wanted to do this for 16 years!)
3. Hmm, hadn't thought about this. What did you have in mind?
I definitely think it's enough time to cut off new upload for people over the limit, but you should strongly consider whether or not your storage costs are that high as to make it worth it to not give a longer runway for people to export.