From this line:
Please select at least one to upgrade
It seems that you need at least one checkbox checked. If that is the case, you could do something like this:
$("form").submit(function(){
var length = $("input[type=checkbox]").not(":checked").length;
if (length > 1) {
alert("Please select at least one to upgrade.");
return false;
}
});
Which checks to see how many checkboxes are not checked, determines if that number is less than the required number, and if it is, triggers the error message.
You can also further simplify the length
variable if you are comfortable with CSS selectors:
var length = $("input[type=checkbox]:not(:checked)").length;
Here's an example.