There is a discussion about Instagram's approach to migrating to python3 around the 34 minute mark in this talkpython podcast https://talkpython.fm/episodes/show/155/practical-steps-for-... Lisa Guo's presentation starts around the 13 minute mark in the 2017 keynote linked to on that page.

Here's a blog post about Dropbox migrating from python 2 to 3 https://blogs.dropbox.com/tech/2018/09/how-we-rolled-out-one...