There is no ‘click a button’ way to make it default to ‘standard’. You can change line 26 of the meta-box.php file to return the defined constant VAT_GROUP_CLASS_STANDARD instead of VAT_GROUP_CLASS_REDUCED.
You can also create a function to update your downloads to make sure the post meta value ‘_moss_rate_type’ exists for each download and has the value ‘standard’.
But I’m not sure it is worth the effort. The VAT class name is just some text included in the MOSS summary report generated by the plugin. You are going to have to transcribe the text of this report into a format suitable for the MOSS portal provided by your tax authority anyway. So instead of cutting and pasting the output ‘Reduced’ you can paste ‘Standard’.
A MOSS report only has one line per country so even if you sell to buyers across all 28 member states every quarter there will only ever be 28 instances of the word to change. It has taken me longer to write this response than it will take you to change the text for MOSS reporting quarters years into the future.