Blog Post: https://kian.org.uk/progressive-s3-to-cloudflare-r2-migration-using-workers/
This is a Cloudflare Worker that will allow you to progressively migrate files from an S3-compatible object store to Cloudflare R2.
When a file is requested, it will check if it exists in R2 and return it if it does - if not, it'll fetch it from your object store and both return that response instantly whilst pushing it into R2 in the background.
For implementing features such as conditionals or ranges, take a look at this project: https://github.com/kotx/render