Impact of miss-ordered fields in a composite, clustered primary key?
The order of columns in a primary key does not affect the insert performance, since the combination of the values in the two columns of the primary key are meant to be unique.  

However the order of index impacts the performance of SELECT queries. In the new order you will have a greater and increasing range of values (dates) across which the index is clustered (dates) as compared to clustering around 100 stores. 

If most of your queries are focused on a single store with the new ordering your queries will be slower.