Default Value of a Parameter on a SSRS Report will not be updated after it was deployedPosted: July 28, 2016
When you modify default value of a parameter for a deployed report of all SSRS versions,
the value will not be updated for the parameter on the reporting server. This is a designed behavior.
The step you need to do is to delete the report through report manager on the server
and redeploy the report and the default value of the parameter will be updated.