As Gizmodo observed, the most straightforward option is to visit your Flickr Camera Roll and hit "select all" for every photo upload date until you've chosen about 500 photos. From there, you can click "download" and wait for a FlickrMail notification that your download is available as a ZIP file. You may have to do this multiple times if you want absolutely everything, but it will make sure your precious shots won't vanish in to the ether. You can also download whole albums, although that will only work if you've organized the shots you'd like to keep.
It's a sharp contrast from Flickr's strategy under Yahoo (now part of Engadget parent Verizon), when free users had 1TB of ad-supported space. However, Smugmug is betting that the limit will encourage people to submit photos they'd want to share with the world, rather than using Flickr as a dumping ground. It's also hoping that a switch to a more modern AWS-powered framework will attract people tired of the Yahoo era's occasional sluggishness and outages. Whether or not people bite is another story -- it could lead to a tighter-knit community, but it could also lead affected users to switch to free social networks and cloud storage services.