@beachbum1972 Found the blog post about it. There's not a lot of detail there but the tl;dr is to use the nginx try_files directive to check the storage bucket first before the local filesystem.
https://community.hachyderm.io/blog/2022/12/03/leaving-the-basement/#solution-nginx-try_files

@beachbum1972 If you're starting with media on your local system, you can configure your proxy server (nginx or equivalent) to try serving media from DO first and then falling back to the local filesystem. Then configure mastodon to use S3 (all new media will now be in DO). At this point you have all the time you need to migrate the files over from the local filesystem to DO. This is effectively what Hachyderm did when migrating media from local filesystem to a storage bucket.