Is StoreStyleRowCol called whenever the style for a cell is changed? How do I know when value is changed in StoreStyleRowCol and not only font or other attributes?

Yes, StoreStyleRowCol is called whenever the style for a cell is changed. You can check the include-bits of the style passed to StoreStyleRowCol to determine which style-attributes will change.

For example, if you want to know whether the value of a cell will be changed you might check

   if (style.GetIncludeValue())
      ; // Value will be changed
   else 
      ; // Only formatting will be changed