Starting with build 1.1.484, Azure AD Connect introduced a regression bug which requires sysadmin permissions to upgrade the SQL database. This bug is still present in the latest build 1.1.614. If you are upgrading to this build, you will need sysadmin permissions. Dbo permissions are not sufficient. If you attempt to upgrade Azure AD Connect without having sysadmin permissions, the upgrade will fail and Azure AD Connect will no longer function correctly afterwards. Microsoft is aware of this and is working to correct this.
This does not affect customers running localDB instances of SQL. The current version of AAD Connect is 1.1.614.0.
Customers running SQL Server back ends are currently excluded from automatic upgrades, so it always requires a manual upgrade to a newer version of AAD Connect. That means (for now) you don't need to be worried about an auto-upgrade automatically breaking AAD Connect functionality.