You can delete the Application rule instance through Records Explorer-> Application ->Application.
Also you need to manually delete application related access groups, operators,rulesets.

Hi,

My understanding on the updating an application with below approaches.

- If the rules and data migration happened to a new database instance which has 7.3 and application servers are pointed to that database then just clean up the database instance of 7.2 

- If the update happened on the same database then there is no point of decommissioning of an application as the user access groups will be pointed to the latest application which is on Pega 7.3

Please let us know, if we need to consider something.

Thanks

Sandeep P V