Upgrade and custom patch

Good morning,
i need to upgrade to 12 vesrion an enterprise infrastructure composed by an admin and about ten managed server.
Some of the managed server has custom patch sent by support team to resolve different issue.
At the moment of upgrade whats happen? the patch will be maintained? I must deinstall it before upgrade and re-apply?
How i can know if the new version include correction of detected issues and i no longer need the patch?

An example: In 11900 version i receive a patch for MSSQL monitor that expose the column "status" in database details and allow to activate an alarm on it. This correction could be included in the 12 version?

Many thanks