So, it turns out when you move to static blog generation and do the generation on your laptop, which is usually in the timezone you’re currently physically located, it can cause URLs to change. Especially if you’re prone to blogging late at night, which can result in even just a shift to DST changing things. I’ve forced jekyll to UTC by adding timezone: 'UTC' to the config, and ensuring all the posts now have timezones for when they were written (a lot of the imported ones didn’t), so hopefully things should be stable from here on.