<textarea class="content">
    [shortcode1 param1=test param2=john]
    [shortcode2 param1=test param2=john]
    [shortcode1 param1=john param2=exchange]
</textarea>

<textarea class="content">
    [shortcode1 newparam1=test newparam2=john]
    [shortcode2 param1=test param2=john]
    [shortcode1 newparam1=john newparam2=exchange]
</textarea>

$(".content").text($(".content").text().replace("param1", "newparam1"));
$(".content").text($(".content").text().replace("param2", "newparam2"));