Reducing your Downtime while Upgrading to EBS 12.1/12.2
Take a look at Doc ID 1578187.1, it discusses upgrading Oracle E-Business Suite and considerations to reduce downtime.
This white paper is an overview and collections of items to consider by the technical team for the upgrade.
Oracle E-Business Suite Reducing Downtime Upgrading to 12.1/12.2 (Doc ID 1578187.1)