//gets all image links with ref to the "product_images" directory
  
          $j("a[href*='product_images/']").each(function() {
        
               //gets the existing href attribute        
         
               var linkhref = $j(this).attr("href");

               //writes the new link reference to the anchor link element
     
              $j(this).attr("href", linkhref.replace(".png", "a.png"));
          
               newfile = $j(this).attr("href").split('/').pop();
        
              $j(this).attr("href", ("http://www.mywebsite/wp-content/uploads/" + newfile));
     
         });//ends "each" function

I am using $j to call jQuery in no-conflict mode.

Basically I want to just rename some links to a different directory.

I am sure there are much cleaner ways to write this.

For a start, it assumes that the files will always be in .PNG format. But if someone uses a GIF the script breaks.
How to fix that?

Second, is there a more portable way of renaming the new path rather than manually typing the complete path as I have done here?

Perhaps to make it clearer,

the new file has the same file name with the suffix "a" added.
Example

myfile.png

rename to

myfilea.png


The existing file is in the "product_images" directory.
I want to rename it to the "uploads" directory.

Just split the href on the '.' and use the last member of the array to determine what the existing suffix is.