jQuery Grid Plugin - jqGrid

When I create the jqgrid, I use: postData:{SearchConditionStr:'123',MyStore:'234',SearchTimeType:'345'}

and it works well.

 

Now I need to modify the SearchConditionStr to '111'. So I use:

grid.setGridParam({postData:{'SearchConditionStr':'111'}});

 

The SearchConditionStr is changed from 123 to 111 correctly. I want to know if I do this, what about the MyStore and SearchTimeType parameters?  If I need to update only some dat in postData, what's the correct method? 

yours,
Ivan