Not exactly no downtime... but all site content stayed available throughout the move thanks to our handy read-only mode. We added this early in the life of the site and it's proved extremely useful for a couple of major moves. It's a pretty simple implementation (based on our site-wide feature flags) - once in read-only mode all login cookies are ignored (so everyone gets the signed-out experience) and the sign-in button is disabled. We also return an error for any POST requests just in case someone has already loaded a page with a form before we turned read-only mode on.
The moment we've gone in to read-only mode we can create a brand new instance of the site from a copy of the (now frozen) database, make any necessary changes to that, then switch traffic over to the new instance once we've tested that everything is working properly. If the new version of the site has a problem we can turn read-only mode off and continue to run on our original database.
If you're running a more content-oriented site it's well worth taking the time to set this kind of thing up - it's not too hard to do, and it gives you an enormous amount of flexibility for maintenance further down the line.
The moment we've gone in to read-only mode we can create a brand new instance of the site from a copy of the (now frozen) database, make any necessary changes to that, then switch traffic over to the new instance once we've tested that everything is working properly. If the new version of the site has a problem we can turn read-only mode off and continue to run on our original database.
If you're running a more content-oriented site it's well worth taking the time to set this kind of thing up - it's not too hard to do, and it gives you an enormous amount of flexibility for maintenance further down the line.