Generally speaking, the test cases cover a deployment (which through regression testing should be used for updates of ADDM as well as the OS) for a solution which covers both appliances for scanning, consolidation as well as a synchronization towards a CMDB.
Given that, it seems that the most beneficial places to direct effort would be anywhere that might be particular to your environment, or where it interfaces with your environment.
It’s reasonable to assume for instance that a pair of newly upgraded appliances at the same version will happily consolidate; but if you have some custom patterns deployed, they would certainly warrant checking.
Don’t forget item 1 on the plan is to go through the release notes before even running the upgrade; any foreseen potential issues that might require your action will be noted in there. It could also highlight areas of product change that coincide with areas of deployed customisation.
In an ideal world there would be some sort of ‘deployment document’ for a site, which details what changes are made from a clean appliance. Each one of those customisations could then be accompanied by a test that proves it works. If you can’t define a test that shows a customisation is working when you decide you need the customisation, you probably never can.
For ADDM, much of the product tries to help here too; highlighting non default discovery scripts in the UI for instance, or using baseline to examine changes brought about by an upgrade.
Finally I’d put a calendar entry just far enough into the future to have a look when “at least one of everything” has happened; each scheduled range, etc.