Hi,

there is no officially supported API for the planning component. What was possible - with the described caveats - in the past is explained here: https://rsjazz.wordpress.com/2015/06/11/creating-plans-with-the-plain-java-client-api/

With respect to the work item attributes shown in the plan, this is live data stored in the work items that the plan loads from said work items to display them. You want to change the work items and, if needed, refresh/reload the plan. There is no planning API, let alone one that allows to change work item attributes.