Very nice article, I always love to read technical reports from existing products :)

Since I don't know the details of your infrastructure, before you decided to move to a newer PSQL engine, had you considered splitting the primary database (e.g. with federation)? I guess you anyhow wanted to do the migration eventually for obvious reasons, but with the split the eventual downtime would only affect a partial customer base at the time.

Edit: I found the answer in the HN discussion, thanks :)