How to interchange the left and right column sidebar in magento?

Go to "app/design/frontend/{Your theme}/{Your theme}/layout/"
look in all the XML files for this text: reference="left" and reference="right" and interchange them.

For example Replace

    
        images/media/col_left_callout.jpg
        Our customer service is available 24/7. Call us at (555) 555-0123.
        checkout/cart
    

to

    
        images/media/col_left_callout.jpg
        Our customer service is available 24/7. Call us at (555) 555-0123.
        checkout/cart