Unfortunately, a technical deep dive on the exact parameters of the transition wasn't really in scope for this piece. I tried to include detail where I could, and Evernote has a really wonderful 5-part blog series that goes into many of the details of the transition. (It's now linked in this post.)
To provide some additional details on the transfer: Evernote was going from data centers in California that it owned and operated to Google, not from S3. Technically speaking, the migration isn't 100% complete, as Evernote is still finishing the transfer of its attachment files to Google.
Evernote did not disclose the exact details of how that transfer was executed on the record, and thus it did not end up in my piece. Thanks for reading!