Is the ampersand in the name of the product, the description, or multiple fields?
You can use the str_replace function to do the replacement for you on the fly so that you do not have to update all of the products. If the character is causing problems when it is in the product name then you could add the function into the add to cart server behavior on the details. Near line 111 should be the binding for the item name, if you updated this line of code to look like this it would do the replacement for you:
$ATC_itemName = "".str_replace(" & ", " and ", $row_WADAProducts['ProductName']) ."";// column binding
This would look for the ampersand separated by spaces (" & ") and replace it with " and ". If the ampersand appears differently from this or is in another field for the product let me know and I can help you get it worked out.