Hi, @Michael Howard Welcome to the Microsoft Q&A forum, Thanks for posting your question.
There will be momentary connection resets during the upgrade process, similar to routine maintenance operations. No impact other than that. Basic tier caches will lose data, but Standard/Premium tier caches will not. Yes the connection blip will momentarily close connections that client applications are using to talk to the cache. The Redis client library will automatically restore connections within seconds, the same as a routine server maintenance event so the impact on users should be minimal.
Please let me know if you have any additional questions