You can't manipulate :after, because it's not technically part of the DOM and therefore is inaccessible by any JavaScript. But you can add a new class with a new :after specified.
CSS:
.pageMenu .active.changed:after {
/* this selector is more specific, so it takes precedence over the other :after */
border-top-width: 22px;
border-left-width: 22px;
border-right-width: 22px;
}
JS:
$('.pageMenu .active').toggleClass('changed');
UPDATE: while it's impossible to directly modify the :after content, there are ways to read and/or override it using JavaScript. See "Manipulating CSS pseudo-elements using jQuery (e.g. :before and :after)" for a comprehensive list of techniques.