SSRS Report changing default parameter after publish

Hey there,
I have an SSRS report published to the report site. The base report has a large number of values in one parameter, I want to use the same base report to publish to multiple folders and change the default parameter value after publishing. When I select manage > Parameters I see the parameter it has the use default checkbox checked and then expression based or override. If I choose override I can see the all the values but cannot select.
Anyone have any idea what I am doing wrong?