There are ways to do migrations of S3 backups to Wasabi or BackBlaze, but they are complicated and expensive. For a 3TB client, we took a USB Hard drive MSP360 backup of the client data and brought it to our location where we have 100mbps upstream bandwidth. We then uploaded the backup to BackBlaze (took 7 days), but we were able to keep running the S3 Backups until we were done with the upload. We then connected the client's server to the BackBlaze Bucket - did a repo sync (that took over a day itself) and ran the backups. Since we had 90 days of version retention in the Amazon Bucket, we did not delete the S3 bucket until 90 days after the cutover.
The extra cost was maintaining the two buckets for 3 monthgs plus the tech time to oversee the migration, but very low compared to a Snow/Fireball method.