If you cancel Flickr Pro, the main limit is the free-plan photo/video count, not your current storage size in GB. Based on the community answer, you can keep uploading only if your account has fewer than 1,000 photos/videos. If you have more than that, you won’t be able to add new uploads unless you reduce the total.

Your existing photos do not disappear immediately, but after some time Flickr may begin deleting the oldest items until your account is back down to 1,000 photos/videos. You should still be able to delete photos yourself. The reply was uncertain about editing, so the safest assumption is: don’t rely on full Pro features after cancellation, and back up anything important before downgrading.

In short: under 1,000 items, you can continue uploading on the free plan; over 1,000, new uploads stop, and older photos may eventually be removed starting with the oldest.