With PHP (What slack was using at one point for some of the services. I think everything uses Hack now which may still maintain a similar model). Switching directories can be mostly atomic.
PHP-FPM with opcaching doesn't need to access files once all the opcodes are cached (turn off file modification checks in production). When you move the directory, you will restart the service.
Unless a request hits a file that is rarely used and not cached, you should be not receive any errors moving the directories.
Its not uncommon to in sass stacks to have multiple data stores, each specialized for different use cases. I do see it appealing to have one system that can support each type of use case and able to tune that as needed.
PHP-FPM with opcaching doesn't need to access files once all the opcodes are cached (turn off file modification checks in production). When you move the directory, you will restart the service.
Unless a request hits a file that is rarely used and not cached, you should be not receive any errors moving the directories.