Updating existing products via Drupal Migrate
Took a slightly different approach to solve this. 


Upgraded Migrate module from version 2.5 to 2.6
Ran upgrades drush updatedb, made sure the migration groups were registered (did this via migrate ui)
After reading the following articles regarding the new 'trackchanges' hash value option:
http://timonweb.com/using-hash-value-trackchanges-to-detect-source-data-changes-in-migrate-for-drupal-7
&amp; http://www.acquia.com/blog/migrate-26-framework-changes I now
applied the following to the migrate in step 1):

$this-&gt;source = new MigrateSourceCSV($csvfile, $columns, array(
  'header_rows' =&gt; 1,
  'track_changes' =&gt; 1
));



The use of track changes now allows the same CSV to be uploaded, any new rows will create a product. Any changes will be detected via the hash that this option provides in the migrate_map table - thus causing the particular product to be updated.