Migrating from S3 has been always easy, as long as you agree in paying the awful amount of money AWS is asking you to do it. But using R2 as a lazy loading layer for S3 seems like a very clever trick move that can alleviate the process a lot by spreading the payment in a longer period of time.
And who knows, maybe after some months the remaining data can be considered cold enough to just archive it.
edit: fix sentence.