Hi @SuT 

To make implementation plan, backout plan, and scheduled dates mandatory on the 'New' state in ServiceNow Change Requests,
create a UI Policy with the condition State [is] New.

Within the UI policy actions, set the fields Implementation plan Backout plan, Planned start date, and Planned end date to Mandatory

Note: it seems existing OOB one (on Assess state) will not be impacted as from new state those will be filled up by value.

try.