Welcome to the Dataiku Community.
In my experience doing upgrades in place with Dataiku DSS since 2017 my connections have generally made it through the upgrades. I've used only file system, Postgres, SalesForce, and REST API connections, so I don't know anything about BigQuery. Others will have to jump in on that point. There is details updated documentation this is a good starting point.
Do also check out the release notes:
If you are using a paid license the support team is really helpful. Drop them a support ticket. Backups are recommended. However, I only think that I had to do any significant work after an upgrade once. That was a few years back prior to the pandemic.
Hope that helps.