Basically, yes!

For the backend: our traffic is load balanced between multiple servers, so I can just spin up a new server with the latest version of Lemmy, add it to the load balancer, and then start taking down the servers with older versions. That way, there is no disruption at all for users, because there is always a server available to handle incoming traffic. The only requirement for this is that the new version of Lemmy can’t have database migrations which break the old running servers.

For lemmy-ui: it’s a bit more complicated, because even with a load balancer, it’s still possible that one user making multiple sequential requests can end up getting responses from different servers. This is problematic, because during an upgrade, files from the new version are not available on the old servers, and vice versa. Fortunately, there are many ways to solve this problem, for lemm.ee, the solution I use is to just always serve lemmy-ui files from object storage, for all versions. In other words, after I upload lemmy-ui files for a new version, these will immediately also be available on old servers.